Part 105

The essential part of this Hygrometer consists of two very thin strips of wood, about a foot long and half an inch broad, glued together, in such a manner that the grain or fibres of the one shall be at right angles with the other; so that when this compound strip was placed in erect position, the grain of one of the pieces of wood would have a vertical, and that of the other an horizontal position. One end of this simply constructed instrument is to be made fast to a wall, or plane board, with the edge outward, and the other end is to be at liberty to move.

Then, as moisture has little or no effect on the length of a piece of wood, or in the direction of its fibres, but a very sensible one on its breadth, or transverse direction, especially when thin, it follows, that on any increase of moisture in the air, this Hygrometer becomes bent into a curve, convex on the side of the transverse fibres; and vice versâ. The degrees, from the greatest dryness to the greatest moisture, are to be marked on a curve drawn on the board or wall, described by the motion of the free end of the Hygrometer; and an index, attached to the moving end of it, will point out, on this graduated arch, the existing state of the atmosphere at the moment, in relation to its condition of moisture or dryness: The relative degree of either, on the smallest change from the one to the other, will be indicated with much precision; and probably, with much more uniformity and truth, in the results of long-continued observations, than can be attained to by the use of Hygrometers constructed of metal, or any other substance than wood.

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Footnote A46:

The second volume of the Transactions of the American Philosophical Society contains a letter, written on the 13th of November, 1780, by Dr. Benjamin Franklin, then in France, to Mr. Nairne, of London: but it was not communicated to the Society, until January, 1786.

In that letter, Dr. Franklin suggests to Mr. Nairne (an eminent optician, and mathematical instrument maker,) the idea of an Hygrometer made of wood; in preference to metalline instruments, for the purpose of discovering “the different degrees of humidity in the air of different countries;”—an idea which occurred to the Doctor, in consequence of a casual circumstance, mentioned in his letter.

Dr. Franklin supposed “a quick sensibility of the instrument, to be rather a disadvantage” to it; “since,” says he, “to draw the desired conclusions from it, a constant and frequent observation day and night, in each country—when the design is, to discover the different degrees of humidity in the air of different countries—will be necessary for a year or years, and the mean of each different set of observations is to be found and determined.”—“For these reasons,” continues the Doctor, “I apprehend that a substance which, though capable of being distended by moisture and contracted by dryness, is so slow in receiving and parting with its humidity that the frequent changes in the atmosphere affect it sensibly, and which therefore should, gradually, take nearly the medium of all those changes and preserve it constantly, would be the most proper substance, of which to make an Hygrometer:”—and he believes good mahogany wood to be that substance. In the concluding part of this letter, Dr. Franklin says to his correspondent: “I would beg leave to recommend to you—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 shew, in sensible degrees, by a moveable hand upon a marked scale, the otherwise less sensible quantities of such contraction and dilatation.”

Hence it appears, that Franklin and Rittenhouse conceived an idea of the same kind, nearly at the same time: but that the latter carried his invention into practice, three or four years before the theory of the former, founded on similar principles, had been announced to the American public, or, as it is believed, was made known to any other person than Mr. Nairne. W. B.

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Astronomical Observations, made in the years 1776, 1777 and 1778, at Philadelphia, by the Rev. Dr. W. Smith, and David Rittenhouse, John Lukens, and Owen Biddle, Esquires: copied from a manuscript account of those Observations, drawn up by Dr. Smith; never before published.

ASTRONOMICAL OBSERVATIONS, 1776.

This year exhibiting little else but scenes of confusion and distress amidst the calamities of an unhappy war, scarce any attention was paid, by the members of the American Philosophical Society, to astronomical or any other literary subjects. It was agreed, however, by Mr. Rittenhouse, Mr. Lukens and myself, to look out whether Mercury would touch the Sun’s disc the 2d of November this year; as a very small difference of latitude from what the Tables give, would have carried the planet clear of the Sun: but, from our observation of the transit of this planet, in 1769, we had reason to expect it would pass further on the Sun, than Halley’s Catalogue gives it.

The following were the observations made, viz.

Nov. 2d, 1776. I got ready the two f. reflector with the largest object-glass, and shortest eye-tube, magnifying about 95 times.

At 4^h per clock—No appearance of the planet on the Sun, and did not expect it until about half an hour past 4; but as Mr. Lukens and Mr. Rittenhouse had not yet come to me in the college, I sent to hasten them.

At 4^h 5′ per clock—took my eye from the tube to adjust it, and fix the smoked glass, to give clearer vision, the atmosphere being hazy. Having fixed the smoked glass in the proper place, so as to prevent its sliding or falling with its own weight, and before I had applied my eye to the telescope again, Mr. Rittenhouse came in; and I desired him to see if the focus and dark glass were all suitable to his eye, as they were to mine. I had been about 4′ employed in this adjustment.

At 4^h 9′, Mr. Rittenhouse having put his eye to the tube, immediately called out, that he saw the planet on the Sun.

At 4^h 10′ per clock, we judged ☿ had entered one-third of his diameter on the Sun.

At 4^h 17′, we clearly noted the internal contact of the limbs.

At 4^h 45′, we judged the least distance of the nearest limbs to be rather more than one diameter of ☿; or that the distance of the limbs was 10″. We-did not apply the micrometer to make any measures; as we presumed that we could judge the distance as accurately by the eye, as it could be measured; on account of the haziness of the atmosphere and the small altitude of the Sun. We kept viewing the planet till sun-set, the distance of the limbs continuing so nearly the same, that we could scarce perceive any diminution thereof; though we were sure also, that it did increase above 10″.

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Footnote A47:

In a table (in the 2d vol. of Lalande’s Astronomie,) entitled, “Passages de Mercure sur le Soleil, calculés pour trois siècles par les nouvelles Tables,” the transit of that planet, above referred to, is thus set down by Lalande, at Paris; viz.

Year. Conjunct. Mean Geocentric Mid. Mean Semi-dura. Short. Time. Long. Time dist. 1776. Nov. 2. 9^h10′7″. 7.11°3′36″. 9^h49′53″. 0^h36′42″. 15′43″.A

W. B.

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The following were the Observations made for ascertaining the Going of the Clock, by WILLIAM SMITH.

Equal Altitudes. d h ′ ″ h ′ ″ Nov. 3 9 14 9 2 37 12 } ☉ on Merid. per clock } h ′ ″ 15 44 2 35 35 } or mean noon } 11 55 40 Equat. Correspond. + 14.4 Alt. —-—-—— Correct Noon per Clock 11 55 54.4 4 9 32 48 20 56 } Mean Noon, or ☉ on } 11 56 53 34 33 19 13 } Merid. per. Clock } 36 14 17 31 } Equat. of equal } 37 20 16 23 } Altitudes } + 13.8 14 39 } —-—-—— 40 54 2 12 53 } Correct Noon per Clock 11 57 6.8 7 8 51 9 9 29 } Mean Noon } 12 0 19 52 37 8 0 } per Clock } Equat Eq. Alt. + 12 54 1 3 6 37 } 12 0 19 } —-—-— Cor. Noon 12 0 31 per Clock