London, Ball, Cambridge, and others of Bath, with models whose lengths, deflections, and weight were equal, the chains of each model between the fulcrums were only 9 oz. of wire, their spans were 4 feet 6 inches, their deflections 6 inches, and their platforms were 2 feet. The parallel chain model (old system) broke down on putting 6 sacks of beans on its platform weighing about 13 cwt.; the taper chain model (new system) bore the 6 sacks of beans, 7 sacks of malt weighing 10 cwt., 2 cwt. of iron and 11 men at the same time, all of which did not break it down. In Bristol, Jan. 6, 1838, before Messrs. Protheroe, Guppy, and others, two other models of equal material and dimensions were tried; the parallel chain mo. del bore 1565 lbs., the taper model bore 3681 lbs. Again, in Bristol, January 10, 1838, more trials were made before Messrs. Acraman, Daniels, Hillhouse, and many other of the first merchants of Bristol, Dr. Waldren and many others of Bath, with models of equal material; the parallel chains bore 1456 lbs., the taper chains bore 3696 lbs. Another trial before the same party on the same day was made with models constructed by Mr. Cross of Bristol, unknown to Mr. Dredge, in order to prove that all was far in the former trials; and the result was, the parallel chains bore 2632 lbs., and the taper chains bore 6849 lbs., each model broke on adding more weight, and the wire throughout on the taper principle was reduced one size by the experiments.

Now. my Lord, all I request is, in the event of further repairs or improvements being about to be undertaken of the Menai bridge, that you will allow Mr. Dredge to exhibit some similar experiments before your Lordship, or the Treasury, cr before the Bridge Commissioners, and in the presence of any of the most eminent engineers you may choose to summon; finally, my Lord, Mr. Dredge declares that such is his thorough conviction of the truth of his theory and its facility of execution, that he would gladly undertake, at his own expense and risk the whole of the iron work, if he should be allowed to re-construct it, which he believes he could do, the bridge standing all the time, and that it should be competent to sustain 1000 tons on transit; the superfluous iron of the present bridge he is pretty confident would pay him and give a balance in favour of government.

Questions may after all fairly be put to me to learn why with all these advantages of Mr. Dredge's system, exhibited with so much apparent fairness, has not his principle been at once generally acted upon? Why has he

not been called upon in many cases to execute what he thus promises? Why, if he can build the proposed Clifton bridge as he says he could for one third or less than Mr. Brunel's estimate, is he not called upon to do so? One good reason is obvious: a prudent caution on the part of the public disinclines them to overthrow long established systems, and to oppose or even question the judgment of long known and respected aúthorities; this feeling operates very naturally and happily in philosophy as well as in politics, but it should not in either be carried to the extent of checking the progress of improvement by well considered means; too great a tenacity for old systems may exist in the minds of many persons, though their motives may be good and their minds not illiberal; Mr. Dredge's principle of suspension bridge building completely overthrows the theory and practice of a Telford, a Brunel, whose experience and talents we are bound highly to respect, and to whose genius I readily offer the humble tribute of my admiration. Can we then be surprised that the public should evince some fear and some reluctance hastily to adopt Mr. Dredge's novel princi ple or theory, in substitution of that which has been so long acted upon? They ought therefore to pause, they ought to enquire, if there are any persons about to direct the construction of other suspension bridges; it is a duty they owe to those for whom they may be acting, to examine fully into the merits of a novel system which promises fairly such advantages, before they determine to persist in the further adoption of the present, of the correctness of which the state of the Menai bridge and the vast expenditure it occasions may well create a doubt, independent of the obviously faulty principles on which it is, I think clearly shown to be constructed. No human being was ever exempt from error, and Messrs. Telford. Brunel, and others, must not be considered to be infallible. I have only to add, my dear Lord, that in making this address to you, I have no other motive than the desire of assisting to bring forward genius, and secure for the country the benefit of a most valuable discovery and work of art, which appears to me for want of that encouragement which I think it merits, is in danger, like very many others, of being lost sight of altogether.

I have the honour to be,
My dear Lord,

Your faithful and obedient servant,
WESTERN.

To the Viscount Melbourne.

Sir, The boiler of the Cyclops steam frigate was reported lately in the public prints to be a sufferer from collapses. I am not aware whether in this instance a valve had been provided for such a contingency, or whether the inaction of means relied upon as effectual preventives of collapse, was the cause of the mischief which occurred. In either case, however, by the use of my sensitive warning and safety valve, accidents of this, or any other nature incidental upon internal or external pressure, may effectually be prevented. The advantages which my valve possesses over others which I have seen in use, are these:

That it is contained within the boiler. That it opens inwardly. That its action cannot be sudden, because it commences with the first increment of pressure. That it cannot strike because it need never be in actual contact with its seat. That it has no vibratory or dangerous reciprocating motion as in the common valve. That it may be made to sound a whistle, close a damper, or perform any other action necessary to arrest the too rapid generation of steam. That it can be placed entirely beyond the control of subordinate hands, and be regulated for a voyage without a possibility of derangement.

A warning valve of this description may be made about the size of a ship's compass, or as a safety valve of any di mension, due to the power of the furnace, or capacity of the boiler. A short description will suffice to show the efficacy of the machine.

Fig. 1 is an exterior view; fig. 2 a sectional view before the steam is generated, and fig. 3 a sectional view, showing the valve in action. A cast iron box C forms a receiver for an iron-cased leaden weight B, and sufficient mercury G, to cover it. This receiver has a communication with another but inore shallow cast iron receiver E, fixed so many inches above the lower one, as the required pressure may render necessary.

The lower receiver

C is suspended within the boiler, and is provided with a cover F, so contrived as to allow a free communication for steam without permitting any overflow of mercury. The upper receiver E is outside the boiler, and has a commu

nication with the atmosphere, but is suitably covered. Affixed to the weight B, which floats in the mercury, is a spindle and valve A, which acts in vertical guides, and closes against a seat in the top plate E, which is cast in one with the upper receiver, and which is screwed down to the top of the boiler

a, a.

When there is no pressure of steam, the weight is floated, and the valve takes its seat; the mercury is then level, as shown in fig. 2.

When the required pressure is on, the mercury changes its position, and is partly discharged into the upper receiver, but the valve still floats.

When any surplus pressure occurs beyond this registered point, the mercury is insufficient to float the valve, and it drops quietly from its seat. In fig. 3, H H shows the line of mercury when the pressure of the steam is at 5lbs. to the inch.

The pressure being relieved it is again floated, and the valve is closed. By attaching the spindle to a crank upon the damper, it may open or close it as rerequired. By placing a whistle over the orifice an alarm may be immediately sounded. A reference to the engravings in connection with these explanations will fully explain the machine. The communicating tube may be regulated to form a head of any required degree of

pressure.

While upon the subject of safety-valves, I should be glad to offer a few remarks upon the inefficacy and awkwardness of the instruments now in use for the detection of danger from insufficiency or overcharge of water, I allude to the common gauge cocks and stop cocks fixed (as we may well say) on boilers. These are either set fast, or leaking fast. In the former case the engineer declines the risk of scalding himself by the lowpressure steam in the protracted action of the cock, and the alternative of continually easing it. In the latter case of leakage he applies the effectual remedy of screwing the cone into the first de scribed condition of its sticking place. To remedy these evils and to insure to the engineer at all times an easy and an agreeable monitor, I designed the gauge cocks and stop cocks, of which the following are sketches, and of a pai tern which I supplied to the Grand Juaction Railway Company,

Fig. 8, transverse section of stop cock through the stuffing box.

Fig. 9, plan of the handle.

A is the spindle with valve ground into its bed B, and kept down water-tight by the screw C. This screw works freely round the spindle A, and may be adjusted to any degree of tightness by the key at the end of the handle, which is shown separately in fig. 9.

The advantages of this description of stop cock are briefly these:

It can neither leak nor slack under any pressure. The engineer turns back

the handle, detects both the amount of pressure and the quantity of water; the valve closes as he withdraws his hand. The action is momentary. The stop cock will neither set nor leak under any pressure, and it may be made from half an inch to 3 feet service.

For all steam and hot-water purposes, I have found it very valuable, and can recommend its use.

I am, Sir, your obedient servant,
HENRY R. ABRAHAM.

Torrington-street, Russell-square,
April 14, 1840.

66

THE

Sir,-Your correspondent, Mr. Peterson, (page 683) can hardly have expected a full answer to his question, "Where the engines of the British Queen are to be matched?" seeing, (as the French say) that it would be too bad to occupy the pages of the Mechanics' Magazine with the long catalogue it would be requisite to give. It may suffice, perhaps, to point out to Mr. P. the name of one vessel in whose engines those of the British Queen will find much more than their match, especially since she happens to be engaged in the very same trade as the Queen, and the comparison of their merits is all, therefore, the more easy. I allude (I may say of course) to the Great Western, the first of the transatlantic steamers in speed as well as in date.

Has Mr. Peterson entirely forgotten the first voyage home of the Queen, in which, with all advantages on her side, she was at once outstripped and left out of sight by the Western? Or does he rest his claim to superiority on the third voyage, when the Queen, after exciting the anxious fears of all parties concerned, safely reached her destined port in twentyfour days-very nearly double the usual run of her competitor, and no less than ten days over the time in which it has been done by a sailing-packet, without the aid of steam at all! Miserable, mdeed, were the prospects of Atlantic steaming, could no better performances than those of the Queen be relied upon. Why, she has disappointed the mer cantile world on every voyage since she