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are for the most part dry, when they have continued some time.*
[In the Philosophical Transactions for 1774, p. 128, is a letter from Mr. J. S. Winn, to Dr. Franklin, stating, that since he had first made the observation concerning the south or southwest winds succeeding an aurora, he had found it invariably obtaining in twenty-three instances; and he adds in a note a fresh confirming instance. In reply, Dr. Franklin makes the following conjecture.]
The aurora borealis, though visible almost every night of clear weather in the more northern regions, and very high in the atmosphere, can scarce be visible in England but when the atmosphere is pretty clear of clouds for the whole space between us and those regions; and therefore are seldom visible there. This extensive clearness may have been produced by a long continuance of northerly winds. When the winds have long continued in one quarter, the return is often violent. Allowing the fact so repeatedly observed by Mr. Winn, perhaps this may account for the violence of the southerly winds, that soon follow the appearance of the aurora
on our coasts.
* In one of the copies of this paper, there is a line drawn across this last article.-W. T. F.
This paragraph is not contained in Mr. Vaughan's edition, and was probably not communicated to him by the author. - EDITOR.
FROM JOSEPH PRIESTLEY TO B. FRANKLIN.
Vegetation of Plants in Water.
Calne, 27 September, 1779.
Though you are so much engaged in affairs of more consequence, I know it will give you some pleasure to be informed, that I have been exceedingly successful in the prosecution of my experiments since the publication of my last volume.
I have confirmed, explained, and extended my former observations on the purification of the atmosphere by means of vegetation; having just discovered that the green matter I treat of in my last volume is a vegetable substance, and then that other plants that grow wholly in water have the same property, all of them without exception imbibing impure air, and emitting it, as excrementitious to them, in a dephlogisticated state. That the source of this pure air is the impure air in the water is evident from all the plants giving only a certain quantity of air, in proportion to the water in which they are confined, and then giving more air in fresh water. I also find that the water before the plants have been confined in it yields impure air, and afterwards pure air.
From these observations I conclude, that the reason why my sprigs of mint sometimes failed to purify air, was their not being always healthy in a confined state. Whereas these water plants are as much at their ease in my jars as in the open pond.
I have made many other new observations, but they are chiefly of a chemical nature, and not worth making the subject of a letter; though, when you see an account of them in my next publication, I flatter myself you will think some of them curious and important.
As the expense of my experiments is necessarily considerable, Dr. Fothergill (who, in a very obliging manner, interests himself much in them) has, of his own motion, engaged a few of his friends to contribute about forty pounds per annum to assist me in defraying the expense. Indeed, without this assistance, I must have desisted altogether. While I had but a small family, I suffered myself to be drawn by my success into expenses, that now appear to have been rather imprudent.
I have been made very happy by the communication of your very ingenious paper on the Aurora Borealis, and by several accounts of the good state of your health and spirits. May this long continue.
Yours most sincerely,
FROM PIETRO TURINI TO B. FRANKLIN.
Communicating Intelligence of a new Work on the Method of Erecting Conductors of Lightning.
Venice, 18 September, 1780.
For a long time a most brilliant reputation has made your name resound through all parts of the world. All Europe admires you; and there is no one more deeply penetrated than myself with the esteem and veneration, which are due to you for so many reasons. Your works, which I have read with the greatest avidit and from which I have drawn the most useful instructions, have made me comprehend the immense extent of your rare knowledge and of your talents.
The theory of electricity has fixed, and will fix for ever, the attention of men. So interesting a dis
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
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,
TO EDWARD NAIRNE, OF LONDON.
Proposing a slowly sensible Hygrometer for certain
READ AT A MEETING OF THE AMERICAN PHILOSOPHICAL
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