then die down quite rapidly and become inaudible through a space of from three to four miles, and often would not reappear till the vessel was within a mile of the fog-horn. Often when the sound came to the listener against a moderate wind the fog-horn would become inaudible at a distance of three or four miles, while on other days, when the wind was going with the sound, the listener had to sail away twenty-five miles before the horn ceased to be heard. Observations made at Block Island and Point Judith showed this fact in the following manner: The distance between these fog-horns is seventeen miles, and the sound of one can be distinctly heard at the other when the air is quiet and homogeneous; but if the wind blows from one towards the other the listener at the station from which the wind blows is unable to hear the other horn.

The most remarkable series of Henry's observations was made at Whitehead Station, Maine, situate on a small island about one mile and a half off the coast. The vessel was approaching the station from the south and with the wind. "The belt of silence" was reached and traversed, and then the sound reappeared. This happened whether the vessel was steaming towards or away from the station, the wind remaining all the while southerly. But during these observations on the vessel the sounds of the steamer's whistle were heard without interruption at the station. Now the steamer's course was directed to the other side of the station; and steaming away from the fog-horn and against the wind the whistle at the station was constantly heard by those on the vessel, but those at the station now perceived the steamer's whistle to go into and out of the belt of silence."

These facts demanded explanation, and for a long time remained enigmas to Henry; till one day he met with a short paper by Professor Stokes, of Cambridge, England, in the Proceedings of the British Association for 1857 in which the effect of an upper current in deflecting the wave-surface of sound so as to throw it down upon the ear of the auditor, or directing it upward far above his head, is fully explained." In the Report of the Light-House Board for 1874 Henry says: "The explanation [of these phenomena] as suggested by the hypothesis of Professor Stokes, is founded on the fact that in the case of a deep current of air the lower stratum, or that next the earth, is more retarded by friction than the one immediately above, and this again than the one above

it, and so on. The effect of this diminution of velocity as we descend towards the earth is, in the case of sound moving with the current to carry the upper part of the sound-waves more rapidly forward than the lower part, thus causing them to incline toward the earth or in other words, to be thrown down upon the ear of the observer. When the sound is in a contrary direction to the cur rent, an opposite effect is produced,-the upper portion of the soundwaves is more retarded than the lower, which, advancing more rapidly in consequence, inclines the waves upward and directs them above the head of the observer. To render this more clear, let us recall the nature of a beam of sound, in still air, projected in a horizontal direction. It consists of a series of concentric waves perpendicular to the direction of the beam, like the palings of a fence. Now, if the upper part of the waves has a slightly greater velocity than the lower, the beam will be bent downward in a manner somewhat analogous to that of a ray of light in proceeding from a rarer to a denser medium. The effect of this deformation of the wave will be cumulative from the sound-centre outward, and hence, although the velocity of the wind may have no perceptible effect on the velocity of sound, yet this bending of the wave being continuous throughout its entire course, a marked effect must be produced. A precisely similar effect will be the result, but perhaps in a considerably greater degree, in case an upper current is moving in an opposite direction to the lower, when the latter is adverse to the sound, and in this we have a logical explanation of the phenomenon observed by General Duane, in which a fog-signal is only heard during the occurrence of a northeast snowstorm. Certainly this phenomenon cannot be explained by any peculiarity of the atmosphere as to variability of density, or of the amount of vapor which it may contain.”

Henry's services to the Light-House Board were of great value to the country. The fact that his investigations showed that lard oil when heated to about 250° Fahrenheit is superior in fluidity and illuminating power to sperm oil, caused the substitution of the former for the latter; and thus was saved a dollar on each gallon of illuminating material purchased. This amounted to about one hundred thousand dollars a year in favor of the government.

In light and heat Henry made several interesting investigations which, reluctantly, we are obliged to pass over. One however holds so important a place in the history of science that it cannot

be omitted from any discourse which would treat of Henry as a discoverer. I refer to his application of the thermopile to the determination of the distribution of heat on the optical images of distant objects. It occurred to Henry that images in the foci of mirrors and lenses are formed not alone by converging pencils of light coming from corresponding points of the objects placed before these mirrors and lenses, but that images are also formed by the convergence of rays which have no effect on the optic nerve, such as the rays of heat. Indeed Henry looked upon the image as having, on a small scale, the same distribution of physical actions as exist on the surface of the large object, of which this image is the optical reproduction.

He applied this conception in a bold and wonderful experiment; which was no other than to study the distribution of heat on the surface of the sun. In 1845, in company with his brother-in-law, Professor Stephen Alexander, he formed an image of the sun by pointing a telescope to that body and then drawing out the eye-tube of the instrument till the solar image was clearly defined on a screen. In this screen was cut a small aperture, closed by the surface of a thermopile. By motion of the telescope any part of the solar image could be brought on to the surface of the pile. A solar spot of considerable magnitude being then present, he brought it on to the pile and noticed the amount of deflection produced in the needles of the galvanometer by the thermo-electric current. Then the parts of the sun's image adjacent to the spot were brought to the thermopile; and now he observed a greater deflection in the galvanometer than in the previous experiment; thus "clearly proving," as he says, "that the spot emitted less heat than the surrounding parts of the luminous disk."

This new method of research originated with Henry. It was shown to Secchi while he was in this country as Professor in the College of Georgetown. On his return to Europe, Secchi obtained no inconsiderable repute by extending these observations-using the methods of Henry, but, I fear, not giving sufficient credit to the originator of them. But let that pass; for the bread which Henry cast upon the waters has returned to our own shores-thanks to the genius and perseverance of our colleague Langley. Most reluctantly do I here desist from citing further the works of Henry. It is impossible to crowd into one brief hour the thoughts which were his occupation during more than half a cen

tury. I have at least endeavored to exhibit before you the more important 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 definitions 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 accredit Thackeray, is one of the very best novels ever written in English. After listening to this, you may form your own opinions as to whether Henry did or did not possess 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 distinguishing 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 generally 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 so the word signifies) than discovery or 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 discovered the true essence of two things, without discerning their difference, seems to me hard to conceive. Now this last is the undisputed 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 been seldom or never 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. Each born in humble condition, without any the adventitious aids of position or influence, was destined apparently to mechanical occupation. Faraday was an apprentice to a bookbinder. Henry served in the same capacity under a silver

of

smith. 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 modesty and good manners, a good ground surely in which to plant the germs of the scientific life. Each, by innate force of taste and intellect, was impelled to the pursuit of knowledge under obstacles which would have damped the ardor of ordinary youths. 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 Lectures 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 the age of thirteen, in the year 1804, remaining there eight years, and during the chief part of the time bound books. Now it was in those books, in the hours after work, that I found the beginning of my philosophy. There were two that especially helped me, the Encyclopædia Britannica,' from which I gained my first notions of electricity, and Mrs. Marcet's Conversations on Chemistry,' which gave me my foundation in that science.

"Do not suppose that I was a very deep thinker, or was marked as a precocious person. I was a lively, imaginative person, and could believe in the Arabian Nights' as easily as in the Encyclopædia.' But facts were important to me and saved me. I could trust a fact and always cross-examined an assertion. So when I questioned Mrs. Marcet's book by such little experiments as I could find means to perform, and found it true to the facts as I could understand them, I felt that I had got hold of an anchor in chemical knowledge, and clung fast to it. Thence my deep veneration for Mrs. Marcet-first, as one who had conferred great personal good and pleasure on me; and then as one able to convey the truth and principle of those boundless fields of knowledge which concern natural things to the young, untaught and inquiring

mind.

"You may imagine my delight when I came to know Mrs.