SCIENCE. 
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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 years 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 induced 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 cai-eful 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 
even 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 of 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 practical 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 most 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 proved 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 r 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 w as 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 dollars 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 definilion 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 within 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 
