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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. The 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 mislaid and lost; and I afterwards had another made that fitted.

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.

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 vapor. I afterwards indeed doubted whether it might be just only so far as related to the city of London, where I resided; because there are many causes of moisture in the city air, which do not exist to the same degree in the country; such as the brewers' and dyers' boiling caldrons, and the great number of pots and teakettles continually on the fire, sending forth abundance of vapor ; and also the number of animals who by their breath continually increase it; to which may be added, that even the vast quantity of sea coals, burnt there, do in kindling discharge a 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 that 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.

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 England, the magnets entered and came out with 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 vaneering, 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 electrical experiments there, it was remarkable, that a mahogany table, on which my jars stood under the prime conductor to be charged, would often be so dry, particularly when the wind had been some time at northwest, which with us is a very drying wind, as to isolate the jars, and prevent their being charged till I had formed a communication between their coatings and the earth. I had a like table in London, which I used for the same purpose all the time I resided there; but it was never so dry as to refuse conducting the electricity.

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 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

you would permit them to contract and dilate, and will show, in sensible degrees, by a movable hand upon a marked scale, the otherwise less sensible quantities of such contraction and dilatation. 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 hygrometer hitherto in use. * With great esteem, I am, dear Sir, Your most obedient and most humble servant,



On a new-invented Stove.


Passy, 1 May, 1781. SIR, I did intend, when in London, to have published a pamphlet, describing the new stove you mention, and for that purpose had a plate engraved, of which I send you an impression. But I have since been too much engaged in affairs to execute that intention. Its principle is that of a siphon reversed, operating on air, in a manner somewhat similar to the operation of the common siphon on water. The funnel of the chimney is the longer leg, the vase is the shorter; and as, in the common siphon, the weight of water in the longer

* For the description of a hygrometer constructed by Mr. Nairne, in conformity with these directions, see his letter under the date of December 20, 1783.- Editor,

| Brother to the celebrated French financier and philosopher of the same name. — Editor.

leg is greater than that in the shorter leg; and thus in descending permits the water in the shorter leg to rise, by the pressure of the atmosphere; so in this aërial siphon, the levity of the air in the longer leg being greater than that in the shorter, it rises and permits the pressure of the atmosphere to force that in the shorter to descend. This causes the smoke to descend also, and in passing through burning coals it is kindled into flame, thereby heating more the passages in the iron box whereon the vase which contains the coals is placed ; and retarding at the same time the

1 consumption of the coals.

On the left hand of the engraving you see the machine put together and placed in a niche built for it in a common chimney.* On the right hand the parts (except the vase) are shown separately. should desire a more particular explanation, I will give it to you viva voce, whenever you please. I think with you, that it is capable of being used to advantage in our kitchens, if one could overcome the repugnance of cooks to the using of new instruments and new methods. With great respect,


If you

TO FELIX VICQ D’AZYR. On the Long Relention of Infection in Dead Bodies after Sepulture.

Passy, 20 July, 1781. SIR, I received the letter you some time since did me the honor of writing to me, accompanied with a

Probably referring to Plate XV. in the present volume. — Editor. † Vicq d'Azyr was the physician to the Queen of France, and celebrated for his skill in medicine and his knowledge of science. His works were published in six volumes. — EDITOR. VOL. VI.



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