SCIENCE. 



129 



proper relations necessary between batteries and magnet to get the 

 greatest electro-magnetic effects, his discoveries would appear 

 dwarfed, though yet of excellent workmanship. But did he at this 

 time, 1827 to 1832, know of Ohm's law? I infer that Henry ar- 

 rived at his discoveries independently of such knowledge, and for 

 two-fold reasons. First, Ohm's law was published as late as 1827, 

 in Berlin, and was received almost contemptuously. Henry was 

 unable to read German, and Ohm's papers were first published in 

 English in 1841. Secondly, from the manner in which Henry 

 worked at his problems and viewed his results, I conclude that he 

 had no knowledge of Ohm's laws ; else why should he have been 

 astonished at the effects when his intensity magnet was connected 

 with his intensity battery ? Henry, now in possession of powerful 

 magnets, began to work on another problem. He tried to do the 

 reverse of what he had already done. His magnet was made by 

 the action of the electric current, and he now tried to obtain an elec- 

 tric current from the magnet ; and he succeeded. Henry and Far- 

 aday independently discovered the means of producing an electric 

 current and spark from a magnet. Tyndall speaks ot this experi- 

 mental results as the " Mont Blanc of Faraday's own achieve- 

 ments." A few words now will place Henry in his proper and just 

 relations to these important discoveries. All the informa- 

 tion he had received about Faraday's discovery was the ac- 

 count of Faraday's production of magneto-electricity by the 

 sudden insertion of a magnet into a helix and its sudden 

 withdrawal therefrom. Henry's experiment is entirely different, 

 and certainly was entirely original with him ; but it is essentially 

 identical with another of Faraday's of which Henry had no knowl- 

 edge. Thus it appears that, although Henry cannot be placed on 

 record as the first discoverer of the magneto-electric current, he 

 stands alone as its second independent discoverer. 



Henry's next discovery was that of the induction of a current 

 upon itself, or of the extra current, as it is sometimes called. 

 Here he anticipated Faraday by nearly two ypars and a half 

 in the observation of the fundamental facts. Notwithstanding 

 an explicit disclaimer of Faraday, the credit of this discovery 

 has been generally given to the latter. This is accounted 

 for by the fact that, although Henry anticipated others in 

 his observations, he had not leisure to follow them up to 

 their full explanation until after Faraday had completely unrav- 

 eled their nature. In 1838, after his return from a first visit to 

 Europe, Henry discovered an entirely new class of phenomena in 

 electrical induction. He first showed that an induced current may 

 excite a second irduced current in a neighboring closed conductor, 

 that this last may induce a third current, and so on. These cur- 

 rents Henry styled currents of the first, second, third, etc., orders, 

 and he showed that they alternate in their direction successively. 

 He investigates the difference in these currents as they flow 

 through different resistances. The same phenomena he tracks 

 through the inductive sections of the discharge of the Leyden jar 

 and of the frictional electrical machine, and shows how they differ 

 from those produced by the voltaic battery. These researches are 

 the most finished of Henry's investigations, and will ever be re- 

 garded as models of careful and thorough scientific work. 



Henry had a versatile mind, and did not confine his attention to 

 the study of electricity. His researches in molecular physics, 

 though not extensive, are remarkable. Here his suggestions and 

 methods have stimulated others to follow in the paths which he 

 has pointed out. In 1839 Henry made a curious discovery as to 

 the permeability of lead to mercury. He found mercury would 

 even ascend a lead wire to the height of a yard in a few days. He 

 feven made what might be called syphons of lead, which would 

 nearly empty a vessel of mercury by drawing the fluid over its 

 sides. Subsequently, in 1845, with Mr. Cornelius, he proved that 

 copper, when heated to the melting point of silver, would absorb 

 the latter metal. In 1844 Henry was investigating the nature of 

 the forces acting in liquid films. Studying the tenacity ot the 

 soap-bubble film, although his experiments could only furnish 

 approximate results, they showed that the molecular attraction of 

 water for water is really several hundred pounds to the square 

 inch, and probably equal to the attraction of ice for ice. Another 

 of Henry's investigations, having a practxal bearing, should be 

 more widely known than it is. Among his duties as chairman of 

 the United States lighthouse board was the testing of the various 

 physical properties of the oils submitted to the government for 

 purchase. Fluidity was one of these properties for which it 

 seemed mos' difficult to get reliable tests. Here he very inge- 

 niously applied the theorem of Torricelli, which shows that equal 

 quantities of all liquids of equal fluidity will flow out of an orifice 

 in equal times. Henry found that with different oils the flow of 

 equal quantities differed, the rapidity of flow of sperm oil exceed- 

 ing that of lard oil in the ratio of 100 to 167. Alcohol pioved to 

 be less fluid than water. Henry took a deep interest in acoustics. 

 His additions to this science were chiefly the results of experiments 

 upon fog signals. He made extensive experiments with various 

 sound-producing instruments, and eventually decided in favor of 

 the steam syren fog-horn. He determined that these instruments 

 send their sound farthest when tuned very near to the treble C, 

 and he also showed the uselessness of applying reflectors to them. 

 During eleven years Henry sought to advance the efficiency of our 

 fog signals by experiments in all weathers. Many very puzzling 

 facts were collected. Thus it was observed that ■- sound coming 

 to a mariner against the wind would cease to be audible on the 



deck of his vessel while it continued to be heard at the masthead. 

 It was also observed that upon approaching a fog-horn from a 

 distance the intensity of sound would gradually increase, then die 

 down rapidly, become inaudible through a space of three or four 

 miles, and perhaps not reappear until the vessel was within a mile 

 of the instrument. These facts demanded explanations, and for a 

 long time remained enigmas to Henry, till one day he met with a 

 paper by Professor Stokes, in which the effect of an upper current 

 in deflecting a wave of sound is fully explained. This hypothesis 

 of Stokes Henry was able to apply to the solution of the problems 

 in question. 



Henry's services to the light-house board were of great value to 

 the country. The fact that his investigations showed that lard oil 

 heated to about 2500 Fahrenheit is superior in fluidity and illu- 

 minating power to sperm oil caused the substitution of the former 

 for the latter. A dollar a gallon was saved, which amounts to 

 about one hundred thousand dolla r s a year in favor of the gov- 

 ernment. In light and heat Henry made several investigations 

 which we must pass over. One, however, is so important that 

 it cannot be omitted. I refer to his application of the thermopile 

 in determining the distribution of heat on the optical images of 

 distant objects. In a bold, and wonderful experiment, he sought 

 to study the distribution of heat on the surface of the sun. In 1845, 

 with Stephen Alexander, he formed an image of the sun, by 

 means of a telescope, upon a screen. In this screen was cut an 

 aperture, closed by the surface of a thermopile. By a motion of 

 the telescope, any part of the image could be brought upon the 

 pile. A solar spot being present, he clearly proved that it emitted 

 less heat than the surrounding parts of the luminous disc. This 

 method of research was shown to Secchi. On his return to Eu- 

 rope the latter made no small repute by extending these observa- 

 tions, using Henry's methods, but often, I fear, not giving full 

 credit to the originator. But let that pass, for the bread which 

 Henry cast upon the waters has returned to our own shores, 

 thanks to the genius of our colleague Langley. 



It is impossible to crowd into one brief hour the thoughts which 

 were his occupation during more than half a century. I have at 

 least endeavored to exhibit the more important part of the labors 

 of his life. What shall we think of them ? Surely they are on as 

 high a plane as those of any of his contemporaries, and show as 

 much originality as theirs in their conception — as much skill in 

 their execution. Yet it has been said that Henry was not a man 

 of genius. As I have not been able to find that the philosophers 

 who have the special charge of giving from time to time defini- 

 tions of genius, have been able to come to any satisfactory 

 conclusion among themselves, I will leave their company, and, 

 with your liberty, take my definition from a book which, if we ac- 

 credit Thackeray, is one of the very best, if not the best, novel 

 ever written in English. After listening to this I will allow you to 

 form your own opinions as to whether Henry did or did not pos- 

 sess genius. " By genius I would understand that power, or 

 rather those powers, of the mind which are capable of penetrating 

 into all things wiihin our reach and knowledge, and of distinguish- 

 ing their essential differences. These are no other than invention 

 and judgment, and they are both called by the collective name of 

 genius, as they are of those gifts of nature which we bring with us 

 into the world. Concerning each of which many seem to have 

 fallen into very great errors ; for by invention, I believe, is gener- 

 ally understood a creative faculty, which would indeed prove most 

 romance writers to have the highest pretensions to it ; whereas by 

 invention is meant no more, and the word so signifies, than 

 discovery in finding out ; or, to explain it at large, a quick 

 and sagacious penetration into the true essence of all the objects 

 of our contemplation. This, I think, can rarely exist without the 

 concomitancy of judgment, for how we can be said to have discov- 

 ered the true essence of two things, without discovering their dif- 

 ference, seems to me hard to conceive. Now this last is the undis- 

 puted province of judgment ; and yet some few men of wit have 

 agreed with all the dull fellows in the world in representing these 

 two to have seldom or never been the property of one and the 

 same person." My own judgment, if of any value, would rank the 

 ability of Henry — I do not say his achievements — a little below 

 that of Faraday. Indeed their lives and their manners of working 

 were strangely alike. Faraday was the son of a blacksmith. He 

 once wrote: "I love a smith's shop and anything relating to smith- 

 ery. My father was a smith." Henry's father plied a schooner 

 on the Hudson. Each started in life with moral and benevolent 

 habits, well-developed and healthy bodies, quick and accurate 

 perceptions, calm judgment and self reliance, tempered with mor- 

 ality and good manners — a good ground, surely, in which to plant 

 the germs of the scientific life. Faraday was an apprentice to a 

 bookbinder. Henry served in the same capacity under a black- 

 smith. Each, endowed with a lively imagination, was in his 

 younger days fond of romance and the drama ; and, by a singular 

 similarity of accidents, each had his attention turned to science by 

 a book which chance threw in his way. This work in the case of 

 Faraday was " Mrs. Marcet's Conversations on Chemistry," and 

 the book which influenced Henry's career was "Gregory's Lec- 

 tures on Experimental Philosophy, Astronomy and Chemistry." 

 Of Mrs. Marcet's book Faraday thus writes : — " My Dear Friend, 

 — Your subject interested me deeply every way; for Mrs. Marcet 

 was a good friend to me, as she must have been to many of the 

 human race. I entered the shop of a bookseller and bookbinder at 



