Plate XL. Vol. VI. p. 42 0. D Fig.1 The Arrows represent the general Currents of the Air. The Representation is made only for one Quarter and one plete and full circle, extending perhaps ten degrees from the pole, appear to spectators so placed (who could see only a part of it) in the form of a segment, its chord resting on the horizon, and its arch elevated more or less above it, as seen from latitudes more or less distant, darkish in color, but yet sufficiently transparent to permit some stars to be seen through it? 28. The rays of electric matter issuing out of a body, diverge by mutually repelling each other, unless there be some conducting body near to receive them; and if that conducting body be at a greater distance, they will first diverge, and then converge in order to enter it. May not this account for some of the varieties of figure seen at times in the motions of the luminous matter of the aurora; since it is possible, that, in passing over the atmosphere from the north, in all directions or meridians, towards the equator, the rays of that matter may find, in many places portions of cloudy, region, or moist atmosphere under them, which (being in the natural or negative state) may be fit to receive them, and towards which they may therefore converge; and when one of those receiving bodies is more than saturated, they may again diverge from it, towards other surrounding masses of such humid atmosphere, and thus form the crowns, as they are called, and other figures, mentioned in the histories of this meteor? 29. If it be true, that the clouds which go to the polar regions carry thither the vapors of the equatorial and temperate regions, which vapors are condensed by the extreme cold of the polar regions, and fall in snow or hail; the winds which come from those regions ought to be generally dry, unless they gain some humidity by sweeping the ocean in their way; and, if I mistake not, the winds between the northwest and northeast 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. DEAR SIR, 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. |