that height it is so rare as to be almost a vacuum. The air heated between the tropics is continually rising, and its place is supplied by northerly and southerly winds which come from those cool regions. 6. The light, heated air floating above the cooler and denser, must spread northward and southward, and descend near the two poles, to supply the place of the cooler air which had moved towards the equator. 7. Thus a circulation of air is kept up in our atmosphere as in the room above mentioned. 8. That heavier and lighter air may move in currents of different and even opposite directions, appears sometimes by the clouds that happen to be in these currents, as plainly as by the smoke in the experiment above mentioned. Also in opening a door between two chambers, one of which has been warmed, by holding a candle near the top, near the bottom, and near the middle, you will find a strong current of warm air passing out of the warmed room above, and another of cool air entering it below, while in the middle there is little or no motion. 9. The great quantity of vapor rising between the tropics forms clouds, which contain much electricity. Some of them fall in rain, before they come to the polar regions. 10. If the rain be received in an isolated vessel, the vessel will be electrified; for every drop brings down some electricity with it. 11. The same is done by snow and hail. 12. The electricity so descending in temperate climates, is received and imbibed by the earth. 13. If the clouds are not sufficiently discharged by this means, they sometimes discharge themselves suddenly by striking into the earth, where the earth is fit to receive their electricity. 14. The earth in temperate and warm climates is generally fit to receive it, being a good conductor. 15. A certain quantity of heat will make some bodies good conductors that will not otherwise conduct. 16. Thus wax rendered fluid, and glass softened by heat, will both of them conduct. 17. And water, though naturally a good conductor, will not conduct well when frozen into ice by a common degree of cold; not at all where the cold is extreme. VOL. III. 3 R 18. Snow falling upon frozen ground has been found to retain its electricity; and to communicate it to an isolated body, when after falling, it has been driven about by the wind. 19. The humidity contained in all the equatorial clouds that reach the polar regions, must there be condensed and fall in snow. 20. The great cake of ice that eternally covers those regions may be too hard frozen to permit the electricity, descending with that snow, to enter the earth. 21. It will therefore be accumulated upon that ice. 22. The atmosphere being heavier in the polar regions, than in the equatorial, will there be lower; as well from that cause, as from the smaller effect of the centrifugal force: consequently the distance to the vacuum above the atmosphere will be less at the poles than elsewhere; and probably much less than the distance (upon the surface of the globe) extending from the pole to those latitudes in which the earth is so thawed as to receive and imbibe electricity; the frost continuing to lat. 80. which is 10 degrees or 600 miles from the pole, while the height of the atmosphere there of such density as to obstruct the motion of the electric fluid, can scarce be estimated above [ ] miles. 23. The vacuum above is a good conductor. 24. May not then the great quantity of electricity brought into the polar regions by the clouds, which are condensed there, and fall in snow, which electricity would enter the earth, but cannot penetrate the ice; may it not, I say (as a bottle overcharged) break through that low atmosphere and run along in the vacuum over the air towards the equator, diverging as the degrees of longitude enlarge, strongly visible where densest, and becoming less visible as it more diverges; till it finds a passage to the earth in more temperate climates, or is mingled with their upper air? 25. If such an operation of nature were really performed, would it not give all the appearances of an AURORA BOREALIS? 26. And would not the aurore become more frequent after the approach of winter; not only because more visible in longer nights; but also because in summer the long presence of the sun may soften the surface of the great ice cake, and render it a conductor, by which the accumulation of electricity in the polar regions will be prevented? 27. The atmosphere of the polar regions being made more dense by the extreme cold, and all the moisture in that air being frozen, may not any great light arising therein, and passing through it, render its density in some degree visible during the night-time, to those who live in the rarer air of more southern latitudes? and would it not in that case, although in itself a complete 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 colour, 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 north-west and north-east are, for the most part dry, when they have continued some time. 1 [In the Philosophical Transactions for 1774, p. 122, is a letter from Mr. J. S. Winn, to Dr. Franklin, stating, that since he had first made the observation concerning the south or south-west 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 boreales, though visible almost every night of clear weather in the more northern regions, and very high in the atmosphere, can scarce be 'In one of the copies of this paper there is a line drawn across this last article. 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. 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 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 aerial 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 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. If you should desire a more particular explanation, I will give it to you viva voce, whenever you please. I think with you that ! See Plate IV. 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, I have the honor to be, &c. B. FRANKLIN. ON THE LONG RETENTION OF INFECTION IN DEAD BODIES AFTER SIR, SEPULTURE, &c. To MONS. VICQ D'AZYR. Passy, July 20, 1781. I RECEIVED the letter you some time since did me the honor of writing to me, accompanied with a number of the pieces that were distributed at the last public meeting of the Royal Society of Medicine, I shall take care to forward them to different parts of America, as desired. Be pleased to present my thanks to the society for the copy sent me of the curious and useful reports relating to the sepulture in the island of Malta. I should be glad of another copy, if it can be spared, being desirous of sending one to each of the philosophical societies in America. With respect to the length of time during which the power of infection may be contained in dead bodies, which is considered in that report, I would mention to you three facts, which, though not all of equal importance or weight, yet methinks it may be well to preserve a memorandum of them, that such observations may be made when occasion offers, as are proper to confirm or invalidate them. While I resided in England, I read in a newspaper, that in a country village at the funeral of a woman whose husband had died of the small-pox 30 years before, and whose grave was dug so as to place her by his side, the neighbors attending the funeral were offended with the smell arising out of the grave, occasioned by a breach in the husband's old coffin, and 25 of them were in a few days taken ill with that distemper, which before was not in that village or its neighborhood, nor had been for the number of years above mentioned. About the years 1763 or 1764, several physicians of London, who had been present from curiosity at the dissection of an Egyptian mummy, were soon after taken ill of a malignant fever, of which they died. Opinions were divided |