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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 rerípe for making one. B. FRANKLIN. ceived from you. It is therefore with pro
priety that I address to you the following ac
count of it; and the more, as you have both a Mr. Nairne, London.
head to contrive and a hand to execute the On the properties of an Hygrometer.—Read in means of perfecting it. And I do this with
the Transactions of the American Philosophi- greater pleasure, as it affords me the opportucal Society, January 26, 1780.
nity of renewing that ancient correspondence Passy, near Paris, Nov. 13, 1780. and acquaintance with you, which to me was The qualities hitherto sought in a hygro- always so pleasing and so instructive. meter, or instrument to discover the degrees You may possibly remember, that in or about of moisture and dryness in the air, seem to the year 1758, you made for me a set of artihave been, an aptitude to receive humidity ficial magnets, six in number, each five and a readily from a moist air, and to part with it as half inches long, half an inch broad, and one readily to a dry air. Different substances eighth of an inch thick. These, with two pieces have been found to possess more or less of of soft iron, which together equalled one of this quality ; but when we shall have found the magnets, were enclosed in a little box of the substance that has it in the greatest per- mahogany wood, the grain of which ran with, fection, there will still remain some uncer- and not across, the length of the box; and the tainty in the conclusions to be drawn from the box was closed by a little shutter of the same degree shown by the instrument, arising from wood, the grain of which ran across the box: the actual state of the instrument itself as to and the ends of this shutting piece were be heat and cold. Thus, if two bottles or vessels velled so as to fit and slide in a kind of dove of glass or metal being filled, the one with tail groove when the box was to be shut or cold and the other with hot water, are brought opened. 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 ab- which was made so as to allow sufficient room sorb more moisture, and mark the air as having for the magnets to slide out and in freely, and, become more humid than it is in reality, and when in, afforded them so much play that by the contrary in a change from warm to cold. shaking the box one could make them strike
But if such a suddenly changing instrument the opposite sides alternately, continued in the could be freed from these imperfections, yet same state all the time I remained in England, when the design is to discover the different which was four years, without any apparent degrees of humidity in the air of different coun- alteration. I left England in August 1762, tries, I apprehend the quick sensibility of the and arrived at Philadelphia in October the instrument to be rather a disadvantage ; since same year. In a few weeks after my arrival, to draw the desired conclusions from it, a con- being desirous of showing your magnets to a stant and frequent observation day and night philosophical friend, I found them so tight in in each country will be necessary for a year | the box, that it was with difficulty I got them or years, and the mean of each different set out; and constantly during the two years I of observations is to be found and determined. remained there, viz. till November 1764, this After all which some uncertainty will remain difficulty of getting them out and in continued. respecting the different degrees of exactitude This little shutter too, as wood does not shrink with which different persons may have made lengthways of the grain, was found too long and taken notes of their observations.
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 the same freedom, and when in, I could ratwas become full big enough for my magnets, tle them against its sides; this has continued and too wide for my new shutter; which was to be the case without sensible variation. My 80 much too short for its grooves, that it was habitation is out of Paris distant almost a apt to fall out; and to make it keep in I length league, so that the moist air of the city canened it by adding to each end a little coat of not be supposed to have much effect upon the sealing-wax.
box. I am on a high dry hill, in a free air, I continued in England more than ten years, as likely to be dry as any air in France.and during all that time, after the first change, Whence it seems probable that the air of I perceived no alteration. The magnets had England in general may, as well as that of the same freedom in their box, and the little London, be moister than the air of America, shutter continued with the added sealing-wax since that of France is so, and in a part so disto fit its grooves, till some weeks after my tant from the sea. second return to America.
The greater dryness of the air in America As I could not imagine any other cause for appears from some other observations.
The this change of dimensions in the box, when in cabinet work forinerly sent us from London, the different countries, I concluded, first ge- which consisted in thin plates of fine wood nerally that the air of England was moister glued upon fir, never would stand with us; than that of America. And this I supposed the veneering, as those plates are called, would an effect of its being an island, where every get loose and come off: both woods shrinking, wind that blew must necessarily pass over and their grains often crossing, they were for some sea before it arrived, and of course lick ever cracking and flying. And in my elecup 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.
Now what I would beg leave to recommend When I was in England the last time, you to you, is, that you would recollect, if you can, also made for me a little achromatic pocket the species of mahogany of which you made telescope, the body was brass, and it had a my box, for you know there is a good deal of round case, (I think of thin wood) covered difference in woods that go under that name; with shagreen. All the while I remained in or if that cannot be, that you would take a England, though possibly there might be some number of pieces of the closest and finest small changes in the dimensions of this case, grained mahogany that you can meet with, I neither perceived nor suspected any. There plane them to the thinness of about a line, was always comfortable room for the tele- and the width of about two inches across the scope to slip in and out. But soon after I ar- grain, and fix each of the pieces in some inrived in America, which was in May 1775, strument that you can contrive, which will the case became too small for the instrument, permit them to contract and dilate, and will it was with much difficulty and various con- show, in sensible degrees, by a moveable hand trivances that I got it out, and I could never upon a marked scale, the otherwise less senafter get it in again, during my stay there, sible quantities of such contraction and dilawhich was eighteen months. I brought it tion. If these instruments are all kept in the with me to Europe, but left the case as useless, same place while making, and are graduated imagining I should find the continental air of together while subject to the same degrees France as dry as that of Pennsylvania, where of moisture or dryness, I apprehend you will my magnet box had also returned a second have so many comparable hygrometers, which, time to its narrowness, and pinched the pieces, being sent into different countries, and conas heretofore, obliging me too, to scrape the tinued there for some time, will find and show sealing-wax off the ends of the shutter. there the mean of the different dryness and
I had not been long in France, before I moisture of the air of those countries, and that was surprised to find that my box was be- with much less trouble than by any hygrocome as large as it had always been in Eng. meter hitherto in use. land, the magnets entered and came out with
To Dr. John Pringle.
in this island, I lately put my design of makOn the Difference of Navigation in shoal and lowing manner.
ing the experiment in execution, in the foldeep Water.
I provided a trough of plained boards fourCRAVEN-STREET, May 10, 1768. teen 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 appre
Not having a watch that shows seconds, in hend there might be something in it, and at- order to measure the time taken up by the tempted to account for it from this considera- boat in passing from end to end, I counted as tion, that the boat in proceeding along the fast as I could count to ten repeatedly, keep canal, must in every boat's length of her ing an account of the number of tens on my course, move out of her way a body of water, fingers. And as much as possible to correct equal in bulk to the room her bottom took up any little inequalities in my counting, I rein the water; that the water so moved must peated the experiment a number of times at pass on each side of her and under her bót- each depth of water, that I might take the metom to get behind her; that if the passage dium. And the following are the results. under her bottom was straitened by the shal Water 1} inches deep. lows, more of that water must pass by her Ist exp....... 100.... 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 made many other experiments, but the I should have convenient time and oppor- aboveare those in which I was most exact; and tunity
they serve sufficiently to show that the differAfter our return to England, as often as I ence is considerable. Between the deepest happened to be on the Thames, I inquired of and shallowest it appears to be somewhat our watermen whether they were sensible of more than one fifth. So that supposing large any difference in rowing over shallow or deep canals and boats and depths of water to bear water. I found them all agreeing in the fact, the same proportions, and that four men or that there was a very great difference, but horses would draw a boat in deep water four they differed widely in expressing the quan- leagues in four hours, it would require five to tity of the difference; some supposing it was draw the same boat in the same time as far equal to a mile in six, others to a mile in in shallow water; or four would require five three, &c. As I did not recollect to have met hours. with any mention of this matter in our philo Whether this difference is of consequence sophical books, and conceiving that if the dif- enough to justify a greater expense in deepenference should really be great, it might be an ing canals, is a matter of calculation, which object of consideration in the many projects our ingenious engineers in that way will reanow on foot for digging new navigable canals dily determine
.104 ...106. .100.
99. .100. .100.
79 78 77 .79
Alphonsus Le Roy, Paris.
to the motion of the sail is not apparent to the Improvements in Navigation. Read in the Ame- eye, because the greater force of the wind,
which strikes it in the direction EEE, overrican Philosophical Society, December 2, 1785.
powers its effect and keeps the sail full in the At sea, on board the London Packet, Capt. Truxton. curve a, a, a, a, a. But suppose the wind to
cease, and the vessel in a calm to be impel. Your learned writings on the navigation led with the same swiftness by oars, the sail of the ancients, which contain a great deal of would then appear filled in the contrary curve curious information, and your very ingenious b, b, b, b, b, when prudent men would immecontrivances for improving the modern sails diately perceive, that the air resisted its mo (voilure) of which I saw with great pleasure tion, and would order it to be taken in. a successful trial on the river Seine, have in Is there any possible means of diminishing duced me to submit to your consideration and this resistance, while the same quantity of judgment, some thoughts I have had on the sail is exposed to the action of the wind, and latter subject.
therefore the same force obtained from it? I. 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 to water; and to have given little attention to gether the same quantity of canvass, were the circumstances of her moving through placed as in figure 3, each having one quarter another fluid, the air. It is true that when a of the dimensions of the great sail, and exposvessel sails right before the wind, this cir- ing a quarter of its surface to the wind, would cumstance is of no importance, because the give a quarter of its force; so that the whole wind
goes with her; but in every deviation force obtained from the wind would be the from that course, the resistance of the air is same, while the resistance from the air would something, and becomes greater in proportion be nearly reduced to the space between the as that deviation increases. I wave at pre- pricked lines a b and c d, before the foremost sent the consideration of those different de sail. grees of resistance given by the air to that It may perhaps be doubted whether the repart of the hull which is above water, and sistance from the air would be so diminished; confine myself to that given to the sails; for since possibly each of the following small their motion through the air is resisted by the sails having also air before it, which must be air, as the motion of the hull through the removed, the resistance on the whole would water is resisted by the water, though with be the same. less force, as the air is a lighter fluid. And This is then a matter to be determined by to simplify the discussion as much as possible, experiment. I will mention one that I many I would state one situation only, to wit, that years since made with success for another of the wind upon the beam, the ship's course purpose ; and I will propose another small being directly across the wind : and I would one easily made. If that too succeeds, I suppose the sail set in an angle of 45 degrees should think it worth while to make a larger, with the keel, as in the following figure; in though at some expense, on a river boat; and the Plate, Fig 1.
perhaps time, and the improvements experiA B represents the body of the vessel, C D ence will afford, may make it applicable with the position of the sail, EEE the direction of advantage to larger vessels. 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 way ing or diminishing only as the fire increased what water it meets with between the prick- or diminished, I contrived to place my jack ed lines FF. And the sail, to go forward, so as to receive the current; and taking off must move out of its way all the air its whole the flyers, I fixed in their stead on the same dimension meets with between the pricked pivot a round tin plate of nearly the same lines CG and DG. Thus both the fluids give diameter with the whole; and having cut it resistance to the motion, each in proportion in radial lines almost to the centre, so as to to the quantity of matter contained in the di- have six equal vanes, I gave to each of them mensions to be removed. And though the the obliquity of forty-five degrees. They air is vastly lighter than the water, and there- moved round, without the weight, by the imfore more easily removed, yet the dimension pression only of the current of air, but too being much greater its effect is very con- slowly for the purpose of roasting. I suspectsiderable.
ed that the air struck by the back of each It is true that in the case stated, the re- vane might possibly by its resistance retard sistance given by the air between those lines the motion ; and to try this, I cut each of them