covery, reserved to the sublimity of your genius, must necessarily introduce a sensible difference in the explanation of the principal phenomena of nature, and of many of its laws. Indeed the greater part of the aerial and terrestrial phenomena are explained by the aid of the electric fire with facility and convincing precision.

The useful theory of conductors, a very important fruit of your studies, makes every day some new progress. The unanimous consent of men proves the truth of the system, and its universal adoption in practice shows how generally they are persuaded of the advantages which result from it.

The method at present used throughout the world, in erecting conductors upon edifices, is essentially the same; and if there is any difference, it consists of some trifles, and more or less carefulness, which depend very often upon the ideas of those, who have the charge of their erection. Yet it must be confessed, that this method has some essential faults, which may sooner or later be the cause of irreparable losses. Powder magazines especially remain exposed to the dreadful attacks of lightning, and thus we still have sad consequences to fear from them.

You will find, Sir, in the little work accompanying this, which I have the honor to dedicate to you, an exposition of the reasons which have convinced me of the insufficiency of the precautions generally adopted. I hope I have found the means of perfectly sheltering these very important edifices from lightning, and given generally more security to other buildings. The progress of the human mind is very rarely rapid. On the contrary it advances by small degrees, and men of genius resembling yourself are rare, who can extend their views to the utmost, and fix the true principles of JJ *

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an important theory, like that of which you are the author. I have done nothing more than make a precise application of the best ascertained rules to the most careful practice. It would be mere rashness to pretend much, after your discoveries.

"Da lunge il seguo, e sue vestigie adoro."

I am anxious to submit this essay to your experience, and I shall be much flattered, if it is such as to merit your approbation. It is written in Italian, with the view of instructing my own countrymen. The importance of the matter has emboldened me to take the liberty to send it to you, such as it is. At least I shall have the satisfaction of having seized this opportunity of expressing to you the sentiments of admiration, and of respect, with which I have the honor to be,

Sir, &c.

PIETRO TURINI.

TO EDWARD NAIRNE, OF LONDON.

Proposing a slowly sensible Hygrometer for certain Purposes.

READ AT A MEETING OF THE AMERICAN PHILOSOPHICAL

SIR,

SOCIETY, JANUARY 26TH, 1786.

Passy, near Paris, 13 November, 1780.

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 room will attach itself in quantities to the surface of the cold vessel; while, if you actually wet the surface of the hot vessel, the moisture will immediately quit it, and be absorbed by the same air. And thus, in a sudden change of the air from cold to warm, the instrument 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.

For these reasons 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 have not time to 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 such an hygrometer.

Such an instrument, you, my dear Sir, though without intending it, have made for me; and I, without desiring or expecting it, have received 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.

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 inches and a half 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 bevelled so as to fit and slide in a kind of dovetail groove when the box was to be shut or opened.

I had been of opinion, that good mahogany wood was not affected by moisture so as to change its dimensions, and that it was always to be found as the tools of the workman left it. Indeed the difference at different times in the same country is so small as to be scarcely in a common way observable. Hence the box, 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.. 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,