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piece of hail, one would think could not long | so slow in receiving and parting with its hucontinue to produce the same effect; since the midity, that the frequent changes in the atmoair, through which the drops fall, must soon be sphere have not time to affect it sensibly, and stripped of its previously dissolved water, so which therefore should gradually take nearly as to be no longer capable of augmenting the medium of all those changes and preserve them. Indeed very heavy showers, of either, it constantly, would be the most proper subare never of long continuance; but moderate stance of which to make such an hygrometer. rains often continue so long as to puzzle this Such an instrument, you, my dear sir, though hypothesis: so that upon the whole I think, without intending it, have made for me; and as I intimated before, that we are yet hardly I, without desiring or expecting it, have reripe for making one. B. FRANKLIN. ceived from you. It is therefore with propriety that I address to you the following account of it; and the more, as you have both a head to contrive and a hand to execute the means of perfecting it. And I do this with greater pleasure, as it affords me the opportunity of renewing that ancient correspondence and acquaintance with you, which to me was always so pleasing and so instructive.

Mr. Nairne, London.

On the properties of an Hygrometer-Read in the Transactions of the American Philosophical Society, January 26, 1780.

PASSY, near Paris, Nov. 13, 1780.

You may possibly remember, that in or about the year 1758, you made for me a set of artificial magnets, six in number, each five and a half inches long, half an inch broad, and one eighth of an inch thick. These, with two pieces of soft iron, which together equalled one of the magnets, were enclosed in a little box of mahogany wood, the grain of which ran with, and not across, the length of the box: and the box was closed by a little shutter of the same wood, the grain of which ran across the box. and the ends of this shutting piece were be velled so as to fit and slide in a kind of dove tail groove when the box was to be shut or opened.

THE qualities hitherto sought in a hygrometer, or instrument to discover the degrees of moisture and dryness in the air, seem to have been, an aptitude to receive humidity readily from a moist air, and to part with it as readily to a dry air. Different substances have been found to possess more or less of this quality; but when we shall have found the substance that has it in the greatest perfection, there will still remain some uncertainty in the conclusions to be drawn from the degree shown by the instrument, arising from the actual state of the instrument itself as to heat and cold. Thus, if two bottles or vessels of glass or metal being filled, the one with cold and the other with hot water, are brought into a room, the moisture of the air in the I had been of opinion, that good mahogany room will attach itself in quantities to the sur- wood was not affected by moisture so as to face of the cold vessel, while if you actually change its dimensions, and that it was always wet the surface of the hot vessel, the moist- to be found as the tools of the workman left ure will immediately quit it, and be absorbed it. Indeed the difference at different times in by the same air. And thus, in a sudden change the same country is so small as to be scarcely of the air from cold to warm, the instrument in a common way observable. Hence the box, remaining longer cold, may condense and absorb more moisture, and mark the air as having become more humid than it is in reality, and the contrary in a change from warm to cold. But if such a suddenly changing instrument could be freed from these imperfections, yet when the design is to discover the different degrees of humidity in the air of different countries, I apprehend the quick sensibility of the instrument to be rather a disadvantage; since to draw the desired conclusions from it, a constant and frequent observation day and night in each country will be necessary for a year or years, and the mean of each different set of observations is to be found and determined. After all which some uncertainty will remain respecting the different degrees of exactitude with which different persons may have made and taken notes of their observations.

which was made so as to allow sufficient room for the magnets to slide out and in freely, and, when in, afforded them so much play that by shaking the box one could make them strike the opposite sides alternately, continued in the same state all the time I remained in England, which was four years, without any apparent alteration. I left England in August 1762, and arrived at Philadelphia in October the same year. In a few weeks after my arrival, being desirous of showing your magnets to a philosophical friend, I found them so tight in the box, that it was with difficulty I got them out; and constantly during the two years I remained there, viz. till November 1764, this difficulty of getting them out and in continued. This little shutter too, as wood does not shrink lengthways of the grain, was found too long to enter its grooves, and, not being used, was For these reasons, I apprehend that a sub-mislaid and lost; and afterwards I had another stance which, though capable of being distend- made that fitted. ed by moisture and contracted by dryness, is

In December, 1764, I returned to England,

and after some time I observed that my box was become full big enough for my magnets, and too wide for my new shutter; which was so much too short for its grooves, that it was apt to fall out; and to make it keep in I lengthened it by adding to each end a little coat of sealing-wax.

I continued in England more than ten years, and during all that time, after the first change, I perceived no alteration. The magnets had the same freedom in their box, and the little shutter continued with the added sealing-wax to fit its grooves, till some weeks after my second return to America.

the same freedom, and when in, I could rattle them against its sides; this has continued to be the case without sensible variation. My habitation is out of Paris distant almost a league, so that the moist air of the city cannot be supposed to have much effect upon the box. I am on a high dry hill, in a free air, as likely to be dry as any air in France.Whence it seems probable that the air of England in general may, as well as that of London, be moister than the air of America, since that of France is so, and in a part so distant from the sea.

The greater dryness of the air in America appears from some other observations. The cabinet work formerly sent us from London, which consisted in thin plates of fine wood glued upon fir, never would stand with us; the veneering, as those plates are called, would get loose and come off: both woods shrinking, and their grains often crossing, they were for ever cracking and flying. And in my elec

As I could not imagine any other cause for this change of dimensions in the box, when in the different countries, I concluded, first generally that the air of England was moister than that of America. And this I supposed an effect of its being an island, where every wind that blew must necessarily pass over some sea before it arrived, and of course lick up some vapour. I afterwards indeed doubt-trical experiments there, it was remarkable, ed whether it might be just only so far as re- that a mahogany table, on which my jars stood lated to the city of London, where I resided; under the prime conductor to be charged, because there are many causes of moisture in would often be so dry, particularly when the the city air, which do not exist to the same wind had been some time at north-west, which degree in the country; such as the brewers with us is a very drying wind, as to isolate and dyers boiling caldrons, and the great the jars, and prevent their being charged till number of pots and tea-kettles continually on I had formed a communication between their the fire, sending forth abundance of vapour; coatings and the earth. I had a like table in and also the number of animals who by their London, which I used for the same purpose breath continually increase it; to which may all the while I resided there; but it was nebe added, that even the vast quantity of sea ver so dry as to refuse conducting the eleccoals burnt there, do in kindling, discharge a tricity. great deal of moisture.

When I was in England the last time, you also made for me a little achromatic pocket telescope, the body was brass, and it had a round case, (I think of thin wood) covered with shagreen. All the while I remained in England, though possibly there might be some small changes in the dimensions of this case, I neither perceived nor suspected any. There was always comfortable room for the telescope to slip in and out. But soon after I arrived in America, which was in May 1775, the case became too small for the instrument, it was with much difficulty and various contrivances that I got it out, and I could never after get it in again, during my stay there, which was eighteen months. I brought it with me to Europe, but left the case as useless, imagining I should find the continental air of France as dry as that of Pennsylvania, where my magnet box had also returned a second time to its narrowness, and pinched the pieces, as heretofore, obliging me too, to scrape the sealing-wax off the ends of the shutter.

Now what I would beg leave to recommend to you, is, that you would recollect, if you can, the species of mahogany of which you made my box, for you know there is a good deal of difference in woods that go under that name; or if that cannot be, that you would take a number of pieces of the closest and finest grained mahogany that you can meet with, plane them to the thinness of about a line, and the width of about two inches across the grain, and fix each of the pieces in some instrument that you can contrive, which will permit them to contract and dilate, and will show, in sensible degrees, by a moveable hand upon a marked scale, the otherwise less sensible quantities of such contraction and dilation. If these instruments are all kept in the same place while making, and are graduated together while subject to the same degrees of moisture or dryness, I apprehend you will have so many comparable hygrometers, which, being sent into different countries, and continued there for some time, will find and show there the mean of the different dryness and moisture of the air of those countries, and that with much less trouble than by any hygro

I had not been long in France, before I was surprised to find, that my box was become as large as it had always been in Eng-meter hitherto in use. land, the magnets entered and came out with

B. FRANKLIN.

To Dr. John Pringle.

in this island, I lately put my design of making the experiment in execution, in the fol

On the Difference of Navigation in shoal and lowing manner. deep Water.

CRAVEN-STREET, May 10, 1768.

I provided a trough of plained boards fourteen feet long, six inches wide and six inches You may remember, that when we were deep, in the clear, filled with water within travelling together in Holland, you remark- half an inch of the edge, to represent a canal, ed, that the trackschuyt in one of the stages I had a loose board of nearly the same length went slower than usual, and inquired of the and breadth, that, being put into the water, boatman, what might be the reason; who might be sunk to any depth, and fixed by litanswered, that it had been a dry season, and the tle wedges where I would choose to have it water in the canal was low. On being ask- stay, in order to make different depths of waed if it was so low as that the boat touch- ter, leaving the surface at the same height ed the muddy bottom; he said, no, not so low with regard to the sides of the trough. I had as that, but so low as to make it harder for a little boat in form of a lighter or boat of burthe horse to draw the boat. We neither of den, six inches long, two inches and a quarus at first could conceive that if there was ter wide, and one inch and a quarter deep. water enough for the boat to swim clear of When swimming, it drew one inch water. Te the bottom, its being deeper would make any give motion to the boat, I fixed one end of a difference; but as the man affirmed it seri- long silk thread to its bow, just even with the ously, as a thing well known among them; water's edge, the other end passed over a well and as the punctuality required in their stages made brass pully, of about an inch diameter, was likely to make such difference, if any turning freely on a small axis; and a shilling there were, more readily observed by them was the weight. Then placing the boat at than by other watermen who did not pass so one end of the trough, the weight would draw regularly and constantly backwards and for- it through the water to the other. wards in the same track; I began to apprehend there might be something in it, and attempted to account for it from this consideration, that the boat in proceeding along the canal, must in every boat's length of her course, move out of her way a body of water, equal in bulk to the room her bottom took up in the water; that the water so moved must pass on each side of her and under her bottom to get behind her; that if the passage under her bottom was straitened by the shallows, more of that water must pass by her sides, and with a swifter motion, which would retard her, as moving the contrary way; or that the water becoming lower behind the boat than before, she was pressed back by the weight of its difference in height, and her motion retarded by having that weight constantly to overcome. But as it is often lost time to attempt accounting for uncertain facts, I determined to make an experiment of this when I should have convenient time and opportunity.

After our return to England, as often as I happened to be on the Thames, I inquired of our watermen whether they were sensible of any difference in rowing over shallow or deep water. I found them all agreeing in the fact, that there was a very great difference, but they differed widely in expressing the quantity of the difference; some supposing it was equal to a mile in six, others to a mile in three, &c. As I did not recollect to have met with any mention of this matter in our philosophical books, and conceiving that if the difference should really be great, it might be an object of consideration in the many projects now on foot for digging new navigable canals

Not having a watch that shows seconds, in order to measure the time taken up by the boat in passing from end to end, I counted as fast as I could count to ten repeatedly, keeping an account of the number of tens on my fingers. And as much as possible to correct any little inequalities in my counting, I repeated the experiment a number of times at each depth of water, that I might take the medium. And the following are the results.

Water 14 inches deep. 2 inches. 1st exp..

2.

.....100...

...104..

104..

4 inches.

.94...

.79

.93.

.78

3...

..91.

.77

4...

[blocks in formation]
[blocks in formation]

I made many other experiments, but the above are those in which I was most exact; and they serve sufficiently to show that the difference is considerable. Between the deepest and shallowest it appears to be somewhat more than one fifth. So that supposing large canals and boats and depths of water to bear the same proportions, and that four men or horses would draw a boat in deep water four leagues in four hours, it would require five to draw the same boat in the same time as far in shallow water; or four would require five hours.

Whether this difference is of consequence enough to justify a greater expense in deepening canals, is a matter of calculation, which our ingenious engineers in that way will readily determine B. FRANKLIN.

Alphonsus Le Roy, Paris. Improvements in Navigation.-Read in the American Philosophical Society, December 2, 1785. At sea, on board the London Packet, Capt. Truxton. August 1785.

YOUR learned writings on the navigation of the ancients, which contain a great deal of curious information, and your very ingenious contrivances for improving the modern sails (voilure) of which I saw with great pleasure à successful trial on the river Seine, have induced me to submit to your consideration and judgment, some thoughts I have had on the latter subject.

to the motion of the sail is not apparent to the which strikes it in the direction EEE, overeye, because the greater force of the wind, powers its effect and keeps the sail full in the curve a, a, a, a, a. But suppose the wind to cease, and the vessel in a calm to be impelled with the same swiftness by oars, the sail would then appear filled in the contrary curve b, b, b, b, b, when prudent men would immediately perceive, that the air resisted its motion, and would order it to be taken in.

Is there any possible means of diminishing this resistance, while the same quantity of sail is exposed to the action of the wind, and therefore the same force obtained from it? I.

placed as in figure 3, each having one quarter of the dimensions of the great sail, and exposing a quarter of its surface to the wind, would give a quarter of its force; so that the whole force obtained from the wind would be the same, while the resistance from the air would be nearly reduced to the space between the pricked lines a b and c d, before the foremost sail.

Those mathematicians, who have endea-think there is, and that it may be done by divoured to improve the swiftness of vessels, by viding the sail into a number of parts, and calculating to find the form of least resist- placing those parts in a line one behind the ance, seem to have considered a ship as a other; thus instead of one sail extending from body moving through one fluid only, the C to D, figure 2, if four sails containing towater; and to have given little attention together the same quantity of canvass, were the circumstances of her moving through another fluid, the air. It is true that when a vessel sails right before the wind, this circumstance is of no importance, because the wind goes with her; but in every deviation from that course, the resistance of the air is something, and becomes greater in proportion as that deviation increases. I wave at present the consideration of those different degrees of resistance given by the air to that part of the hull which is above water, and confine myself to that given to the sails; for their motion through the air is resisted by the air, as the motion of the hull through the water is resisted by the water, though with less force, as the air is a lighter fluid. And to simplify the discussion as much as possible, I would state one situation only, to wit, that of the wind upon the beam, the ship's course being directly across the wind: and I would suppose the sail set in an angle of 45 degrees with the keel, as in the following figure; in the Plate, Fig 1.

It may perhaps be doubted whether the resistance from the air would be so diminished; since possibly each of the following small sails having also air before it, which must be removed, the resistance on the whole would be the same.

This is then a matter to be determined by experiment. I will mention one that I many years since made with success for another purpose; and I will propose another small one easily made. If that too succeeds, I should think it worth while to make a larger, though at some expense, on a river boat; and perhaps time, and the improvements experience will afford, may make it applicable with advantage to larger vessels.

A B represents the body of the vessel, C D the position of the sail, EEE the direction of the wind, MM the line of motion. In observing Having near my kitchen chimney a round this figure it will appear, that so much of the hole of eight inches diameter, through which body of the vessel as is immersed in the water was a constant steady current of air, increasmust, to go forward, remove out of its waying or diminishing only as the fire increased what water it meets with between the pricked lines FF. And the sail, to go forward, must move out of its way all the air its whole dimension meets with between the pricked lines CG and DG. Thus both the fluids give resistance to the motion, each in proportion to the quantity of matter contained in the dimensions to be removed. And though the air is vastly lighter than the water, and therefore more easily removed, yet the dimension being much greater its effect is very considerable.

It is true that in the case stated, the resistance given by the air between those lines

or diminished, I contrived to place my jack so as to receive the current; and taking off the flyers, I fixed in their stead on the same pivot a round tin plate of nearly the same diameter with the whole; and having cut it in radial lines almost to the centre, so as to have six equal vanes, I gave to each of them the obliquity of forty-five degrees. They moved round, without the weight, by the impression only of the current of air, but too slowly for the purpose of roasting. I suspected that the air struck by the back of each vane might possibly by its resistance retard the motion; and to try this, I cut each of them

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