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FROM EDWARD NAIRNE TO B. FRANKLIN.

A Hygrometer on a New Construction. M. De Luc's

Hygrometer.

London, 2 December, 1783. DEAR SIR, I received your favors. The book and prints, which Mr. Argand was so obliging as to deliver, I have since sent to Sir Joseph Banks, agreeably to your request. I am very much obliged to you for your observations on the alteration of the wood of the box belonging to the magnets. Since I received your favor, I have been endeavouring to make as simple an instrument as possible for a hygrometer, a drawing of which is annexed.

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In Fig. 1, A is a piece of wood about twelve inches long and two inches broad, cut crosswise the grain of

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VOL. VI.

the wood, which slides freely between the two pieces of wood, B, B, forming grooves for it. C is a screw for adjusting the piece of wood, A, so that the index may point to the proper division when first made. In Fig. 2, a is a slit to admit the pin e to move freely, which pin, by being fast in the piece of wood, A, moves with it as it shortens or lengthens, and, by pressing against the short end of the index, D, causes it to move up or down according as the weather is moist or dry, which is shown on the divided arch, at the upper end of the instrument.* If

you should at your leisure consider it, and if you find that it is not adequate to the purpose wanted, I should esteem it a great favor if you would inform me of it, or if any alterations or additions can be added to it.t

I was the day before yesterday at Windsor, where M. De Luc showed me a hydrometer. It was made of a thin piece of whalebone, about nine inches long, and was kept straight by a line fastened to it going over a pulley, the other end of which line was fastened to a spiral spring. He observed, that it was the only substance he had ever met with, that would always return to the same length when soaked in water. It altered in its length about one inch from extreme moisture to extreme dryness.

I sent you some time ago the account of shortening wire by lightning, which I hope you have received. I am, dear Sir, your most obliged, &c.

EDWARD NAIRNE.

# It is to be ed, that Fig. 1. is the back part of the instrument, or a thin box upon which is placed Fig. 2, as a cover, being the face of the instrument. - EDITOR.

+ See the letter, containing directions for making this hygrometer above, p. 426.

TO DAVID RITTENHOUSE.

On the Comet seen in Yorkshire.

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Passy, 15 December, 1783. SIR, All astronomical news that I receive, I think it my duty to communicate to you. The following is just come to hand, in a letter from the President of the Royal Society, dated at London the 9th instant.

“A miserable comet made its appearance to Mr. Nathan Pigot, in his observatory at Yorkshire, on the 19th past, and the weather has been so hazy in the evenings that it has scarce been observed since. It was on the 19th

Right Ascen. North Dec.

at 11 15 41 00 3° 10 6 On the 20th 10 54 40 00 4 32

“On the 21st it was seen in the place where it was expected; but the night was too hazy to observe it.

“ It appears like a nebula, with a diameter of about two minutes of a degree; the nucleus faint. It is seen with difficulty when the wires of the instrument are illuminated, but is not visible with an open glass.”

Mr. Pigot. “Nov. 29th. It was seen near the chin of Aries, and appeared like a nebulous star; as there was some moonlight, it was difficult to find it.

« Dec. 1st. It was removed near the preceding eye of Aries; but, conceiving other astronomers, who had fixed instruments, have noted its place, he has not calculated the distance from any known star.”

Mr. Herschell. With great esteem, I have the honor to be, &c.

B. FRANKLIN.

TO JOHN INGENHOUSZ.

On Balloons, and their Probable Importance.

Passy, 16 January, 1784. DEAR FRIEND, I have this day received your favor of the 2d instant. Every information in my power, respecting the balloons, I sent you just before Christmas, contained in copies of my letters to Sir Joseph Banks. There is no secret in the affair, and I make no doubt that a person coming from you would easily obtain a sight of the different balloons of Montgolfier and Charles, with all the instructions wanted ; and, if you undertake to make one, I think it extremely proper and necessary to send an ingenious man here for that purpose; otherwise, for want of attention to some particular circumstance, or of not being acquainted with it, the experiment might miscarry, which, in an affair of so much public expectation, would have bad consequences, draw upon you a great deal of censure, and affect your reputation. It is a serious thing to draw out from their affairs all the inhabitants of a great city and its environs, and a disappointment makes them angry. At Bordeaux lately a person pretended to send up a balloon, and had received money from many people, but not being able to make it rise, the populace were so exasperated that they pulled down his house, and had like to have killed him.

It appears, as you observe, to be a discovery of great importance, and what may possibly give a new turn to human affairs. Convincing sovereigns of the folly of wars may perhaps be one effect of it; since it will be impracticable for the most potent of them to guard his dominions. Five thousand balloons, capable of raising two men each, could not cost more than five ships of the line; and where is the prince who can afford so to cover his country with troops for its defence, as that ten thousand men descending from the clouds might not in many places do an infinite deal of mischief, before a force could be brought together to repel them? It is a pity that any national jealousy should, as you imagine it may, have prevented the English from prosecuting the experiment, since they are such ingenious mechanicians, that in their hands it might have made a more rapid progress towards perfection, and all the utility it is capable of affording.

The balloon of Messrs. Charles and Robert was really filled with inflammable air. The quantity being great, it was expensive, and tedious filling, requiring two or three days and nights constant labor. It had a soupape, or valve, near the top, which they could open by pulling a string, and thereby let out some air when they had a mind to descend; and they discharged some of their ballast of sand when they would rise again. A great deal of air must have been let out when they landed, so that the loose part might envelope one of them; yet, the car being lightened by that one getting out of it, there was enough left to carry up the other rapidly. They had no fire with them. That is used only in M. Montgolfier's globe, which is open at bottom, and straw constantly burnt to keep

This kind is sooner and cheaper filled; but must be of much greater dimensions to carry up the same weight; since air rarefied by heat is only twice as light as common air, and inflammable air ten times lighter. M. Morveau, a famous chemist at Dijon, has discovered an inflammable air that will cost only a twenty-fifth part of the price of what is made by oil

it up.

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