He gave very close study to the philosophy of waterspouts and whirlwinds and came to the conclusion that they were generated by the same causes, and were of the same nature, "the only Difference between them being, that the one passes over Land, the other over Water." He was the first person to discover that northeast storms did not begin in the northeast at all. The manner in which he did it is another good illustration of his quickness in noting the significance of every fact by which his attention was challenged. He desired to observe a lunar eclipse at nine o'clock in the evening at Philadelphia, but his efforts were frustrated by a northeast storm, which lasted for a night and a day, and did much damage all along the Atlantic coast. To his surprise he afterwards learnt from the Boston newspapers that the eclipse had been visible there, and, upon writing to his brother for particulars, was informed by him that it had been over for an hour when the storm set in at Boston; though it was apparently fair to assume that the storm began sooner at Boston than at Philadelphia. This information and further inquiry satisfied him that northeast storms commence southward and work their way to the northeast at the rate of a hundred miles an hour. When we read the words in which he stated his theory of such storms, we begin to understand what Sir Humphry Davy meant in saying that science appeared in Franklin's language in a dress wonderfully decorous, and best adapted to display her native loveliness.
Suppose [he said to Jared Eliot] a great tract of country, land and sea, to wit, Florida and the Bay of Mexico, to have clear weather for several days, and to be heated by the sun, and its air thereby exceedingly rarefied. Suppose the country northeastward, as Pennsylvania, New England, Nova Scotia, and Newfoundland, to be at the same time covered with clouds, and its air chilled and condensed. The rarefied air being lighter must rise, and the denser air next to it will press into its place; that will be followed by the next denser air, that by the next, and so on. Thus, when I have a fire in my chimney, there is a current of air constantly flowing from the door to the chimney; but the beginning of the motion was at the chimney, where the air being rarefied by the fire rising, its place was supplied by the cooler air that was next to it, and the place of that by the next, and so on to the door. So the water in a long sluice or mill-race, being stopped by a gate, is at rest like the air in a calm; but as soon as you open the gate at one end to let it out, the water next the gate begins first to move, that which is next to it follows; and so, though the water proceeds forward to the gate, the motion which began there runs backward, if one may so speak, to the upper end of the race, where the water is last in motion.
It may be truly said of every province of scientific research into which Franklin ventured that he brought to it a bold and original spirit of speculation which gave it new interest and meaning. Even when he was not the first to kindle a light, he had a happy and effective way of trimming it anew and freshening its radiance. To Collinson he wrote on one occasion, "But I must own I am much in the Dark about Light." But noonday is not more luminous than what he had to say on the subject in this letter.
May not all the Phaenomena of Light [he asked] be more conveniently solved, by supposing universal Space filled with a subtle elastic Fluid, which, when at rest, is not visible, but whose Vibrations affect that fine Sense the Eye, as those of Air do the grosser Organs of the Ear? We do not, in the Case of Sound, imagine that any sonorous Particles are thrown off from a Bell, for Instance, and fly in strait Lines to the Ear; why must we believe that luminous Particles leave the Sun and proceed to the Eye? Some Diamonds, if rubbed, shine in the Dark, without losing any Part of their Matter. I can make an Electrical Spark as big as the Flame of a Candle, much brighter, and, therefore, visible farther, yet this is without Fuel; and, I am persuaded no part of the Electric Fluid flies off in such Case to distant Places, but all goes directly, and is to be found in the Place to which I destine it. May not different Degrees of Vibration of the above-mentioned Universal Medium occasion the Appearances of different Colours? I think the Electric Fluid is always the same; yet I find that weaker and stronger Sparks differ in apparent Colour; some white, blue, purple, red; the strongest, White; weak ones, red. Thus different Degrees of Vibration given to the Air produce the 7 different Sounds in Music, analagous to the 7 Colours, yet the Medium, Air, is the same.
"Universal Space, as far as we know of it," he declared in his Loose Thoughts on a Universal Fluid, "seems to be filled with a subtil Fluid, whose Motion, or Vibration is called Light." And he then proceeds to found on this statement a series of speculations marked by too high a degree of temerity to have much scientific value. One sentiment in the paper, however, is well worth recalling as showing how clearly its author had grasped the conservation of matter. "The Power of Man relative to Matter," he observed, "seems limited to the dividing it, or mixing the various kinds of it, or changing its Form and Appearance by different Compositions of it; but does not extend to the making or creating of new Matter, or annihilating the old."
The Science of Palæontology was in its infancy during the lifetime of Franklin. Many years before Cuvier gave the name of mastodon to the prehistoric beast, whose fossil remains had been brought to sight from time to time in different parts of the world, George Croghan, the Indian trader, sent to Franklin a box of tusks and grinders, which had been found near the Ohio, and which he supposed to be parts of a dismembered elephant. In his reply of thanks, Franklin observed that the tusks were nearly of the same form and texture as those of the African and Asiatic elephant. "But the grinders differ," he added, "being full of knobs, like the grinders of a carnivorous animal; when those of the elephant, who eats only vegetables, are almost smooth. But then we know of no other animal with tusks like an elephant, to whom such grinders might belong." The fact that, while elephants inhabited hot countries only, fragments such as those sent to him by Croghan were found in climates like those of the Ohio Territory and Siberia, looked, Franklin concluded, "as if the earth had anciently been in another position, and the climates differently placed from what they are at present." Contrasting the observations of this letter with the paper read long afterwards by Thomas Jefferson before the American Philosophical Society on the bones of a large prehistoric quadruped resembling the sloth, William B. Scott, the American palæontologist, remarks:
Franklin's opinions are nearer to our present beliefs than were Jefferson's, written nearly forty years later. Of course, we now know that Franklin was mistaken in supposing that such bones were found only in what is now Kentucky and in Peru, and his comparison of the teeth of the mastodon with the "grinders of a carnivorous animal" is not very happy, but the inferences are remarkably sound, when we consider the state of geological knowledge in 1767.
In a letter to Antoine Court de Gébelin, the author of the Monde Primitif, Franklin gave him a valuable caution, in relation to apparent linguistic variations. Strangers, who learnt the language of an Indian nation, he said, finding no orthography, formed each his own orthography according to the usual sounds given to the letters in his own language. Thus the same words of the Mohawk language, written by an English, a French and a German interpreter, often differed very much in the spelling.
Franklin's letters to Herschel, Maskelyne, Rittenhouse, Humphrey Marshall and James Bowdoin reveal a keen interest in astronomy, but this is not one of the fields from which he came off cum laude. Gratifying to the pride of an American, however, is an observation which he made to William Herschel, when the latter sent to him for the American Philosophical Society a catalogue of one thousand new nebulæ and star-clusters and stated at the same time that he had discovered two satellites, which revolved about the Georgian planet. In congratulating him on the discovery, Franklin said:
You have wonderfully extended the Power of human Vision, and are daily making us Acquainted with Regions of the Universe totally unknown to mankind in former Ages. Had Fortune plac'd you in this part of America, your Progress in these Discoveries might have been still more rapid, as from the more frequent clearness of our Air, we have near one Third more in the year of good observing Days than there are in England.