But his discovery of

among the minor discoverers. the nature of lightning was so striking and so capable of arousing the wonder of the masses of mankind, and his invention of the lightning-rod was regarded as so universally valuable, that he has received more popular applause than men whose achievements were greater and more important.

During the rest of his life his work in science was principally in the way of encouraging its study. He was always observing, collecting facts, and writing out his conclusions. The public business

in which he was soon constantly employed, and the long years of his diplomatic service in England and France, were serious interruptions, and during the last part of his life it was not often that he could steal time for that loving investigation of nature which after his thirtieth year became the great passion of his life.

His command of language had seldom been put to better use than in explaining the rather subtle ideas and conceptions in the early development of electricity. Even now after the lapse of one hundred and fifty years we seem to gain a fresher understanding of that subject by reading his homely and beautiful explanations; and modern students would have an easier time if Franklin were still here to write their text-books. His subsequent letters and essays were many of them even more happily expressed than the famous letters on electricity.

In old editions of his works all his writings on science were collected in one place, so that they could be read consecutively, which was rather better

umes.

than the modern strictly chronological plan by which they are scattered throughout eight or ten large volAs we look over one of the old editions we feel almost compelled to begin original research at once, it seems so easy and pretty. There are long investigations about water-spouts and whirlwinds,— whether a water-spout ever actually touches the surface of the sea, and whether its action is downward from the sky or upward from the water. He interviewed sea-captains and received letters from people in the West Indies to help him, and those who had once come within the circle of his fascination were never weary of giving aid.

He investigated what he called the light in seawater, now called phosphorescence. The cause of the saltness of the sea and the existence of masses of salt or salt-mines in the earth he explained by the theory that all the water of the world had once been salt, for sea-shells and the bones of fishes were found, he said, on high land; upheavals had isolated parts of the original water, which on evaporation had left the salt, and this being covered with earth, became a salt-mine. This explanation was given in a letter to his brother Peter, and is really a little essay on geology, which was then not known by that or any other name, but consisted merely of a few scattered observations.

Many of his most interesting explanations of phenomena appear in letters to the young women with whom he was on such friendly terms. Indeed, it has been said that he was never at his best except when writing to women. People believe, he tells

Miss Stevenson, that all rivers run into the sea, and he goes on to show in his most clever way that some rivers do not. The waters of the Delaware, for example, and the waters of the rivers that flow into Chesapeake Bay, probably never reach the ocean. The salt water backing up against them twice a day acts as a dam, and their fresh water is dissipated by evaporation. Only a few, like the Amazon and the Orinoco, are known to force their fresh water far out on the surface of the sea. In this same letter he describes the experiments he made to prove that dark colors absorb more of the sun's rays,

and are therefore warmer than white.

While representing Pennsylvania in England, and living with Mrs. Stevenson, in Craven Street, London, he made an experiment to prove that vessels move faster in deep than in shallow water. This was generally believed by seafaring men; but Franklin had a wooden trough made with a false bottom by which he could regulate the depth of water, and he put in it a little boat drawn by a string which ran over a pulley at the end of the trough, with a shilling attached for a weight. In this way he succeeded in demonstrating a natural law which, though known to practical men, had never been described in books of science.

He took much pains to collect information about the Gulf Stream. This wonderful river in the ocean has been long known, but the first people to observe it closely were the Nantucket whalemen, who found that their game was numerous on the edges of it, but was never seen within its warm

waters. In consequence of their more exact knowledge they were able to make faster voyages than other seamen. Franklin learned about it from them, and on his numerous voyages made many observations, which he carefully recorded. He obtained a map of it from one of the whalemen, which he caused to be engraved for the general benefit of navigation on the old London chart then universally used by sailors. But the British captains slighted it, and this, like his other efforts in science, was first appreciated in France.

He has been called the discoverer of the temperature of the Gulf Stream; but this statement is somewhat misleading. That the stream was warmer than the surrounding ocean seems to have been long known; but Franklin was the first to take its temperature at different points with a thermometer. He did this most systematically on several of his voyages, even when suffering severely from sea-sickness, and thus suggested the use of the thermometer in investigating ocean currents. He first took these temperatures in 1775, and the next year Dr. Charles Blagden, of the British army, took them while on the voyage to America with troops to suppress the Revolution. He and Franklin are ranked together as the first to show the value of an instrument which is now universally used in ocean experiments as well as in the practical navigation of ships.*

In the same careful manner he collected all that was known of the effect of oil in stilling waves by

* Pillsbury's Gulf Stream, published by the U. S. government.

making the surface so smooth and slippery that the wind cannot act on it. So fascinated was he with this investigation that he had a cane made with a little receptacle for oil in the head of it, and when walking in the country in England experimented on every pond he passed. But it would be long to tell of all he wrote on light and heat, the vis inertia of matter, magnetism, rainfall, evaporation, and the aurora borealis.

One of the discomforts of colonial times, when large open fireplaces were so common, was a smoky chimney. Franklin's attention was drawn to this question about the time that he invented the Pennsylvania fireplaces, and he made an exhaustive study of the nature of smoke and heated air. He became very skilful in correcting defects in the chimneys of his friends' houses, and while he was in England noblemen and distinguished people often sought his aid. It was not, however, until 1785, near the close of his life, that he put his knowledge in writing in a letter to Dr. Ingenhausz, physician to the Emperor of Austria. The letter was published and extensively circulated as the best summary of all that was known on this important question. It is as fresh and interesting to-day as when it was written, and well worth reading, because it explains so charmingly the philosophy of some phenomena of common occurrence which modern books of science are not at much pains to make clear.

His enemies, of course, ridiculed him as a chimney doctor, and his friends have gone to the other extreme in implying that he was the only man in the world