Page:Popular Science Monthly Volume 14.djvu/86

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76
THE POPULAR SCIENCE MONTHLY.

The student will soon find that different persons, in singing the same note, as nearly alike as they can, will produce flames of very different forms. This is because the voices differ in the number and relative intensities of the simple sounds which form them.

 
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THE RADIOMETER.
By J. W. PHELPS.

SOME twenty-five years ago, when Foucault's ingenious experiment for proving the earth's motion on its axis was in vogue, the idea occurred to us that that fact might be proved in another way. Foucault's method, it will be remembered, consisted in the vibration of the pendulum in a fixed direction, the earth's motion being disclosed by the angular deviation of a given chalked line from that direction. In the pursuit of our own method we conceived the idea—which, though a very simple one, was not more simple than some others have been of experimentalists, both before and after the fact—that, if a small needle, say of dry wood, could be suspended from its middle by a torsionless thread, and be excluded from the air, it would retain any fixed direction, while a parallel line under it would change from that direction in proportion as the horizon turned from west to east. In order to carry out this idea we suspended a wooden needle by a thread of spider's web from the underside of the cork stopper of a large glass jar, and for additional security against possible currents of air placed the whole inside of a large chest.

On going to this chest to ascertain the result of our experiment, which we happened to do by night, and had to take a light with us, we were surprised to see, the moment we raised the lid, the needle begin to move! Our first thought was that we had made a great discovery that light was a material substance, and that enough of that substance could emanate from one small candle to move a needle when freely suspended, in an horizontal position! The weight of light, of course, we knew must be infinitely small, if it had any weight at all; but then, by multiplying what little weight it might have by its known amazing great velocity, we did not know but that the motion which we witnessed might be produced in the way supposed. A little examination, however, into the matter, soon convinced us that our first impressions were erroneous.

We preserved the glass jar with the needle suspended in it for many months; and the most astonishing thing about it to us was that, however much the needle turned, though at times it would spin round with great and long-continued velocity, the thread of spider's web never