As an example of the application of formula (C), we may apply it to the

same case.

We have found that the cover should be 23 inches to make the steam be cut off at three-quarters of the stroke. By formula (C) we may now verify our calculation, and — supposing the valve to be made with a cover of 2} inches-we can see whether or not it will cut the steam off at the proper part of the stroke.

Supposing the piston to be moving downwards, when the steam is cut off, the valve is below its middle position by the length of the cover 23 inches, and when at its middle position, it is half the length of its stroke (5 inches) below its highest position; hence s'=7, or s=7375. The values of the other letters will be the same as before. We will have, therefore, -1 r' — s' COS. -475=1181° or 24110 дов

COS.

The second value of s is evidently the one required for our present pur. pose, and shows that the steam will be cut off at 44.76 inches from the beginning of the stroke, which agrees very nearly with our former calculation, which made a cover of 23 inches cut the steam off at 45 inches from the beginning of the stroke.

1

With regard to the above calculation it may be remarked, that there are an infinite number of arcs which have their cosines equal to 475; but if we were to take more of these arcs it would be found, that they would ultimately all give one or other of the results we have already obtained, so that it is unnecessary to take more than the first two of them.

The reason that we get two values for s, is this. For each position of the valve there are two corresponding positions of the piston; the one being its corresponding position when going downwards, and the other when going upwards. The second value we got for s, therefore, shows us that when the piston is moving upwards, the valve will begin to open the port, a, when the piston is 057 inch from the top of its stroke. By increasing the cover on the steam side of the valve, any amount of expansion may be obtained, and we might thus obviate the necessity of using expansion-valves, were it not that increasing the cover on the steam side beyond moderate limits deranges the working of the exhausting ports. A valve with much cover on the steam side must always shut the exhausting port considerably before the piston has reached the end of the stroke, while it at the same time opens a passage to the condenser for the steam that is acting expansively, and thereby entirely removes the propelling power before the piston has completed its stroke. In the valve we have taken as an example, the operation of this kind of valve in producing expansion is carried too far, or at least to the utmost allowable limits. As another example of the use of formula (C) we shall apply it to the same valve to ascertain to what extent the objections we have just described apply in this instance. Let us suppose that the cover on the exhausting side (c') is ¦ inch, we would then have the exhaustion below the piston cut off when the valve is 5 inches below its highest position.

We thus see that the exhaustion below the piston would be cut off when the piston is still about 6 inches from the bottom of its stroke. The other value of s shows that, when the piston is going upwards, the exhausting passage for the steam below it would be opened when the piston was still 3.03 inches from the top of the stroke.

In what precedes we have spoken exclusively of the common short slidevalve, such as is represented in the figure; but the long D valve works on exactly the same principle; only in it the exhausting sides of the ports are usually reversed. The steam generally enters the cylinder from the insides of the ports, that is, from K and M, and exhausts at F and G.

The principal results of the foregoing observations may be expressed in the four following practical rules, applicable alike to short slide and long D valves.

RULE I. To find how much cover must be given on the steam side in order to cut the steam off at any given part of the stroke.

From the length of the stroke of the piston, subtract the length of that part of the stroke that is to be made before the steam is cut off. Divide the remainder by the length of the stroke of the piston, and extract the square root of the quotient. Multiply the square root thus found by half the length of the stroke of the valve, and from the product take half the lead, and the remainder will be the cover required.

RULE II.-To find at what part of the stroke any given amount of cover on the steam side will cut off the steam.

Add the cover on the steam side to the lead; divide the sum by half the length of stroke of the valve. In a table of natural sines find the arc whose sine is equal to the quotient thus obtained. To this arc add 90°, and from the sum of these two arcs subtract the arc whose cosine is equal to the cover on the steam side divided by half the stroke of the valve. Find the cosine of the remaining arc, add 1 to it, and multiply the sum by half the stroke of the piston, and the product is the length of that part of the stroke that will be made by the piston before the steam is cut off.

RULE III. To find how much before the end of the stroke, the exhaustion of the steam in front of the piston will be cut off.

To the cover on the steam side add the lead, and divide the sum by half the length of the stroke of the valve. Find the arc whose sine is equal to the quotient, and add 90° to it. Divide the cover on the exhausting side by half the stroke of the valve, and find the arc whose cosine is equal to the quotient. Subtract this arc from the one last obtained, and find the cosine of the remainder. Subtract this cosine from 2, and multiply the remainder by half the stroke of the piston. The product is the distance of the piston from the end of its stroke when the exhaustion is cut off.

RULE IV. To find how far the piston is from the end of its stroke, when the steam that is propelling it by expansion is allowed to escape to the condenser.

To the cover on the steam side add the lead, divide the sum by half the stroke of the valve, and find the arc whose sine is equal to the quotient. Find the arc whose cosine is equal to the cover on the exhausting side, divided by half the stroke of the valve. Add these two arcs together, and subtract 90°. Find the cosine of the residue, subtract it from 1, and multiply the remainder by half the stroke of the piston. The product is the distance of the piston from the end of its stroke, when the steam that is propelling it is allowed to escape to the condenser. In using these rules all the dimensions are to be taken in inches, and the answers will be found in inches also.

From an examination of the formulas we have given on this subject, it will be perceived (supposing that there is no lead) that the part of the stroke where the steam is cut off, is determined by the proportion which the cover on the steam side bears to the length of the stroke of the valve: so that in all cases where the cover bears the same proportion to the length of the stroke of the valve, the steam will be cut off at the same part of the stroke of the piston.

In the first line, accordingly, of Table I., will be found eight different parts of the stroke of the piston designated; and directly below each, in the second line, is given the quantity of cover requisite to cause the steam to be cut off at that particular part of the stroke. The different sizes of the cover are given in the second line, in decimal parts of the length of the stroke of the valve; so that, to get the quantity of cover corresponding to any of the given degrees of expansion, it is only necessary to take the decimal in the second line, which stands under the fraction in the first, that marks that degree of expansion, and multiply that decimal by the length you intend to make the stroke of the valve. Thus suppose you have an engine in which you wish to have the steam cut off when the piston is a quarter of the length of its stroke from the end of it, look in the first line of the table, and you will find in the third column from the left,. Directly under that, in the second line, you have the decimal, 250. Suppose that you think 18 inches will be a convenient length for the stroke of the valve, multiply the decimal 250 by 18, which gives 4. Hence we learn, that with an 18-inch

stroke for the valve 4 inches of cover on the steam side will cause the steam to be cut off when the piston has still a quarter of its stroke to perform.

=

Half the stroke of the valve must always be at least equal to the cover on the steam side added to the breadth of the port; consequently, as the cover, in this case, must be 4 inches, and as half the stroke of the valve is 9 inches, the breadth of the port cannot be more than (9-4) 44 inches. If this breadth of port is not enough, we must increase the stroke of the valve; by which means we shall get both the cover and the breadth of the port proportionally increased. Thus, if we make the length of valvestroke 20 inches, we shall have for the cover 250 × 20=5 inches, and for the breadth of the port 10-5=5 inches.

24

23

6.94 6.79

23

6.65

221

22

211

21

201

20

19

19

181

18

6.48 6'00 5:47 4.90 4.25 3:47 2.45 6.34 5.88 5:36 4.79 4.16 3.39 2:39 6.21 5.75 5.24 4.69 4:07 3.32 2.34 6.50 6:07 5.62 5.13 4.59 3.98 3.25 2-29 6.36 5.94 5.50 5'02 4:49 3.89 3.13 2.24 6.21 5.80 5:38 4.90 4:39 3.80 3.10 2.19 6.07 5.67 5.25 4.79 4.28 3.72 3.03 2.14 5.92 5.53 5.12 4'67 4.18 3.63 2.96 2.09 5.78 5:40 5:00 4:56 4.08 3.54 2.89 2:04 5.64 5.26 4.87 4'45 3.98 3:45 2.82 1.99 5:49 5.13 4.75 4:33 3.88 3.36 2.74 1.94 5.34 4.99 4.62 4.22 3.77 3.27 2.67 1.88 5.20 4.86 4.50 4.10 3.67 3.19 2.60 1.83

valve. Suppose that, in the last example, the valve was to have inch of lead, we would subtract inch from the 5 inches found for the cover by the table that would leave 43 inches for the quantity of cover that the valve ought to have.

Table II. is an extension of Table I., for the purpose of obviating, in most cases, the necessity of even the very small degree of trouble required in multiplying the stroke of the valve by one of the decimals in Table I. The first line of Table II, consists, as in Table I., of eight fractions, indicating the various parts of the stroke at which the steam may be cut off. The first column on the left hand consists of various numbers that represent the different lengths that may be given to the stroke of the valve, diminishing, by half-inches, from 24 inches to 3 inches. Suppose that you wish the steam cut off at any of the eight parts of the stroke indicated in the first line of the table (say at from the end of the stroke,) you find at the top of the sixth column from the left. Look for the proposed length of stroke of the valve (say 17 inches) in the first column on the left. From 17, in that column, run along the line towards the right, and in the sixth column, and directly under the at the top, you will find 347, which is the cover required to cause the steam to be cut off at from the end of the stroke, if the valve has no lead. If you wish to give it lead (say inch,) subtract the half of that, or = 125 inch from 3:47, and you will have 3'47 '125 = 3·345 inches, the quantity of cover that the valve should have.

=

To find the greatest breadth that we can give to the port in this case, we have, as before, half the length of stroke, 84-3.345 5.155 inches, which is the greatest breadth we can give to the port with this length of stroke. It is scarcely necessary to observe that it is not at all essential that the port should be so broad as this; indeed, where great length of stroke in the valve is not inconvenient, it is always an advantage to make it travel farther than is just necessary to make the port full open; because, when it travels further, both the exhausting and steam ports are more quickly opened, so as to allow greater freedom of motion to the steam.

The manner of using this table is so simple, that we need not trouble the reader with more examples. We pass on, therefore, to explain the use of Table III.

Suppose that the piston of a steam-engine is making its downward stroke, that the steam is entering the upper part of the cylinder by the upper steam-port, and escaping from below the piston by the lower exhaustingport; then, if (as is generally the case) the slide-valve has some cover on the steam side, the upper port will be closed before the piston gets to the bottom of the stroke, and the steam above then acts expansively, while the communication between the bottom of the cylinder and the condenser still continues open, to allow any vapour from the condensed water in the cylinder, or any leakage past the piston, to escape into the condenser; but, before the piston gets to the bottom of the cylinder, this passage to the condenser will also be cut off by the valve closing the lower port. Soon after the lower port is thus closed, the upper port will be opened towards the condenser, so as to allow the steam that has been acting expansively to escape. Thus, before the piston has completed its stroke, the propelling power is removed from behind it, and a resisting power is opposed before it, arising from the vapour in the cylinder, which has no longer any passage open to the condenser. It is evident, that if there is no cover on the exhausting side of the valve, the exhausting port before the piston will be closed, and the one behind it opened, at the same time; but, if there is any cover on the exhausting side, the port before the piston will be closed before that behind it is opened; and the interval between the closing of the one, and the opening of the other, will depend on the quantity of cover on the exhausting side of the valve. Again, the position of the piston in the cylinder, when these ports are closed and opened respectively, will depend on the quantity of cover that the valve has on the steam side. If the cover is large enough to cut the steam off when the piston is yet a considerable distance from the end of its stroke, these ports will be closed and opened at a proportionably early part of the stroke; and when it is attempted to obtain great expansion by the slide-valve alone, without an expansion-valve, considerable loss of power is incurred from this cause.

Table III. is intended to show the parts of the stroke where, under any given arrangement of slide-valve, these ports close and open respectively, so that thereby the engineer may be able to estimate how much of the efficiency of the engine he loses, while he is trying to add to the power of the steam by increasing the expansion in this manner. In the table, there are eight double columns, and at the heads of these columns are eight fractions as before, representing so many different parts of the stroke at which the steam may be supposed to be cut off.

In the left-hand single column in each double one, are four decimals which represent the distance of the piston (in terms of the length of its stroke) from the end of its stroke when the exhausting-port before it is opened, corresponding with the degree of expansion indicated by the fraction at the top of the double column and the cover on the exhausting side opposite to these decimals respectively in the left-hand column. The righthand single column in each double one contains also each four decimals, which show in the same way at what part of the stroke the exhaustingport behind the piston is opened. A few examples will, perhaps, explain this best.

Suppose we have an engine in which the slide-valve is made to cut the steam off when the piston is 1-3d from the end of its stroke, and that the cover on the exhausting side of the valve is 1-8th of the whole length of its

*055

⚫055

⚫033 ⚫033

⚫022 ⚫022 ⚫011 ⚫011

1-8th

178

⚫033

•161

⚫026

*143

1-16th

•130

090.

118

610.

•126

*052 •100 ⚫040 *085 '030

1-32d 113 ⚫073

•101

990.

*092 ⚫082 stroke. Let the stroke of the piston be 6 feet, or 72 inches. We wish to know when the exhausting port before the piston will be closed, and when the one behind it will be opened. At the top of the left-hand double column, the given degree of expansion (1-3d) is marked, and in the extreme left column we have at the top the given amount of cover (1-8th). Opposite the 1-8th, in the first double column, we have 178 and 033, which decimals, multiplied respectively by 72, the length of the stroke, will give the required positions of the piston: thus 72 x 178-12.8 inches distance of the piston from the end of the stroke when the exhausting port before the piston is shut; and 72 × '033=2·38 inches-distance of the piston from the end of its stroke when the exhausting-port behind it is opened.

=

To take another example. Let the stroke of the valve be 16 inches, the cover on the exhausting side inch, the cover on the steam side 3 inches, the length of the stroke of the piston 60 inches. It is required to ascertain all the particulars of the working of this valve. The cover on the exhausting side is evidently of the length of the valve stroke.

⚫043 ⚫043 Again, looking at 16 in the left-hand column of Table II., we find in the same horizontal line 3.26, or very nearly 31 under at the head of the column, thus showing that the steam will be cut off at from the end of the stroke. Again, under at the head of the fifth double column from the left in Table III., and in a horizontal line within the left-hand column, we have '053 and 033. Hence, 053 × 60=3·18 inches=distance of the piston from the end of its stroke when the exhausting port before it is shut, and 033 × 60-1.98 inches=distance of the piston from the end of its stroke when the exhausting-port behind it is opened. If in this valve the cover on the exhausting side were increased (say to 2 inches, or of the stroke,) the effect would be to make the port before the valve be shut sooner in the proportion of 109 to 053, and the port behind it later in the proportion of 008 to 033 (see Table III.). Whereas, if the cover on the exhausting side were removed entirely, the port before the piston would be shut and that behind it opened at the same time, and (see bottom of fifth double column, Table III.) the distance of the piston from the end of its stroke at that time would be '043 × 60=2.58 inches.

An inspection of Table III. shows us the effect of increasing the expansion by the slide-valve in augmenting the loss of power occasioned by the imperfect action of the eduction passages. Referring to the bottom line of the Table, we see that the eduction passage before the piston is closed, and that behind it opened, (thus destroying the whole moving power of the engine,) when the piston is 092 from the end of its stroke, the steam being cut off at from the end. Whereas, if the steam is only cut off at from the end of the stroke, the moving power is not withdrawn till only 011 of the stroke remains uncompleted. It will also be observed that increasing the cover on the exhausting side has the effect of retaining the action of the steam longer behind the piston, but it at the same time causes the eduction port before it to be closed sooner.

A very cursory examination of the action of the slide-valve is sufficient to show that the cover on the steam side should always be greater than on the exhausting side. If they are equal, the steam would be admitted on one side of the piston at the same time that it was allowed to escape from the other; but universal experience has shown that when this is the case a very considerable part of the power of the engine is destroyed by the resistance opposed to the piston, by the exhausting steam not getting away to the condenser with sufficient rapidity. Hence we see the necessity of the cover on the exhausting side being always less than the cover on the steam side; and the difference should be the greater the higher the velocity of the piston is intended to be, because the quicker the piston moves the passage for the waste steam requires to be the larger, so as to admit of its getting away to the condenser with as great rapidity as possible. In locomotive or other engines, where it is not wished to expand the steam in the cylinder at all, the slide-valve is sometimes made with very little cover on the steam side: and in these circumstances, in order to get a sufficient

Fig. 163.

difference between the cover on the steam and exhausting sides of the valve, it may be necessary not only to take away all the cover on the exhausting side, but to take off still more, so as to make both exhausting passages be, in some degree, open, when the valve is at the middle of its stroke. This, accordingly, is sometimes done in such circumstances as we have described; but, when there is even a small degree of cover on the steam side, this plan of taking more than all the cover off the exhausting side ought never to be resorted to, as it can serve no good purpose, and will materially increase an evil we have already explained; viz. the opening of the exhausting-port behind the piston before the stroke is nearly completed. The Tables apply equally to the common short slide three-ported valves and to the long D valves.

We here introduce the boilers of the noted steam vessels Her Majesty and Royal Consort. These boilers supply a pair of engines with steam, the cylinders of which are 65 inches diameter, and the stroke 5 feet 6 inches. There are four boilers in each vessel, fired at opposite ends, 13 inches separate in the direction of the vessel's length, and 2 feet 6 inches asunder athwart-ships. The flues of the four boilers join so as to form the foot of the funnel, which is 5 feet 6 inches diameter, and thus make it commence in the boiler instead of making it commence clear of the boiler, which is often done by maintaining the flues separate and distinct. Each boiler is 9 feet 11 inches in length, and 9 feet 1 inches in breadth. The height to the top of the steam space is 11 feet 9 inches, and the length of the tubes is 6 feet 2 inches, the diameter 3 inches, and 4 inches distant from centre to centre. The number of tubes is 160 in each. The bottom waterspaces are stayed with fifteen two-inch rivet-headed bolts and nuts, and five two-inch stays secured by nuts pass through across the boiler, between the furnace and tubes. The water-space at the inner side of each boiler is

Fig. 164.

also secured by four two-inch bolts. Each boiler has three furnaces, the centre one being 2 feet 2 inches wide and the side ones 2 feet 4 inches on the outside, but widening within to 2 feet 4 inches, and 2 feet 6 inches respectively. This is a very well secured boiler, but it would be an improvement if the tubes were placed at a slight inclination, in order to facilitate the disengagement of the steam from the aftermost tube plate.

The durability of boilers is affected by many circumstances, the more prominent of which it may be expedient to enumerate. In marine boilers the steam-chests and the ash-pits of the furnaces are the first parts to give way. The outside of the steam-chest is worn away by the dripping of water from the deck, and the inside by the action of the steam, while the wear of the ash-pits is chiefly attributable to the practice of wetting the ashes and quenching the fires with salt water. The action of the steam upon the interior of the steam-chest is most capricious, some parts being worn away rapidly, while other parts are uninjured, and the parts most rapidly deteriorated in one boiler are often untouched in another. The iron of the steam-chest, however, is as a general rule most rapidly worn away in those boilers in which scale is permitted to accumulate, which may perhaps be from the extrication of muriatic acid from the salt. Painting the insides of boilers is not of much use as a preservative from corrosion, and is said to induce priming; but we think a whitewash, if it may so be termed, of Roman cement applied in several successive coats to the interior of the steam-chest, would be found to be an effectual preservative of the iron in that situation. For protecting the outsides of boilers we know of no more effectual method than felting them and covering them afterwards with sheet lead that is soldered wherever there is a joining. The application of felt, however, to the outside of boilers has in some cases been found to accelerate their corrosion in the inside; but the inside corrosion may be obviated by the application of the Roman cement.

Steam-pipes, whether made of iron or copper, are attended with practical objections. Malleable iron pipes become rapidly corroded by the passage of the steam, and large flakes of hard rust are carried into the valve and cylinder, where they work much damage by scratching into ruts the surfaces that ought to be steam-tight. Copper steam-pipes have a galvanic action upon the cylinder faces and the valves, by which the iron is worn away or turned into plumbago. Copper pipes, however, are upon the whole less objectionable than pipes made of iron, and should obtain a preference, though we think it likely that the recent reductions of the duty on glass will lead to the manufacture of pipes enamelled on the inside, which will be greatly superior to those now in use, as they will obviate the evil of corrosion.

All tubular boilers should be provided with a self-acting contrivance for maintaining the feed, and this contrivance should be put in connexion with a small engine whose functions it is to supply the boilers with water. By this expedient the water level may be maintained at its proper height, even when the engine is stationary, without having recourse to the laborious operation of pumping by hand. In a steam vessel a small engine may be applied to a number of useful purposes, besides that of feeding the boiler, 'such as raising the anchor and transferring the coal from one part of the vessel to another.

We have long been of opinion, that although there may not be much prospect of increasing the economy of the present steam engine by obviating sources of waste not yet recognised, yet that the steam engine is not, the best conceivable instrument for obtaining the mechanical power due to a certain quantity of heat. It is only where two different temperatures subsist that mechanical power can be realised, and the farther removed these temperatures are from one another the greater the resulting power must be. That instrument will therefore realise the greatest mechanical effect from a given quantity of heat, which may be worked with the greatest extremes of temperature; and if steam could be employed for working an engine as hot, or nearly as hot, as the fire by which it is generated, the true mechanical effect due to the heat employed would be obtained; but as this obviously cannot be done, there is a loss incidental to the use of the steam engine which other contrivances may be invented to avert. In any case that we can imagine there must be much difficulty in dealing with the high temperatures this principle appears to prescribe, but the principle does not make high temperatures imperative, but merely great differences of temperature; and very low temperatures may not present the same impediments as very high ones. It is not by steam, however, that the benefits due to great differences of temperature are likely to be realised, but by some other agent, that will be better adapted to the new circumstances in which it is placed.

Surcharged steam.-Surcharged steam is steam to which an additional dose of heat has been given after leaving the water, and its temperature is therefore higher than is represented by its elastic force. If from ordinary steam any part of the heat be abstracted, a portion of the steam must be condensed, but some of the heat may be abstracted from surcharged steam without occasioning condensation, and it is therefore spoken of as overcharged with heat, or in other words possessed of more heat than is requisite for maintaining the vaporous form. Much difference of opinion exists as to whether the use of surcharged steam in steam engines is beneficial. The principle we have just mentioned appears to favour the notion that it is, but the general impression among engineers is, that the use of surcharged steam is attended with no advantage. We cannot say we concur in this belief; and as some vindication of our nonconformity, we shall here give

from the Artizan some computations which favour the impression that surcharged steam is productive of an economy in fuel :

(1.) When air is heated it expands, and the increments of volume are proportional to the increments of temperature. Every increment of 1o in temperature produces an increase in volume of th part of the bulk of the air at 32°. This rule has been found to apply to steam out of contact with

water.

(2.) The specific heat of steam out of contact with water is in this case supposed to be inversely as its specific gravity. The specific heat of steam at 2120847.

From these data we may determine the amount of the saving by using the steam surcharged with heat after leaving the boiler. The subject may be investigated as follows; and we may suppose the temperature of the steam to be 212° :

Lett temperature to which the steam is raised out of contact with

Let s

Let

water.

mean specific heat of the steam between the temperatures 2120 and t'.

the volume of steam at the temperature t', the volume at 212° being 100.

Let x=the volume of the same weight of steam at 32°, supposing that it could be cooled to 32° without condensing.

Let h=heat required to raise 100 volumes of steam from 212° to t'.
Let b=weight of water in 100 volumes of steam at 212°.

Let cheat required to raise the temperature of a quantity of water (=b), 1 degree.

Let h' heat required to generate, from water at 60°, a quantity of steam equal in volume to v-100.

in a quantity of steam whose volume is at 212° v-100. Hence, supposing the latent heat of steam to be 1000°, we have

002083 (t −212) __ 2·3954 (t'—212).c h=(1000+150) .cx 1.37494 1.37494

Now, since h=heat required to produce an additional volume of steam equal to v- - 100 by heating the steam out of contact with water, and since h' equals the heat required to make the same addition to the volume of the steam by generating it from water, it follows that the saving of heat by using the former method is h'-h.

.c

2·3954 (C'—212) .C _ { ·847 ("'—212) +

1.37494

which, reduced,