Sir John Herschel, and he gave the explanation that hydrogen breaks the continuity of the medium. But this is not the true explanation. Professor Stokes, paying attention to the fact that when a tuning-fork is struck and held in air it gives out but little sound, investigated the subject, and arrived at the conclusion that air is so mobile that it runs around the tuning-fork without being thrown into waves. Check this "running round" by holding a card at one side of the fork, and the sound is augmented. Now, hydrogen is more mobile still than air, and hence the probable explanation of the bell not sounding in it is, that the hydrogen "runs round" so readily that it is not thrown into waves. Alluding to Newton's attempt to reconcile his theoretical calculation of 916 feet per second with the experimental results of 1,090 feet per second as the velocity of sound. Professor Tyndall said that the philosopher had forgotten to take into account the heat developed by the sound-wave in its own path. By the aid of the thermopile and galvanometer arrangement, the lecturer showed that a very gentle and small compression of air does produce heat. Several experiments to show the passage of sound through wood, water, and other bodies were made in the concluding part of the lecture. In one of these experiments music played in the cellars of the Institution was made audible by a connecting wooden rod rising into the lecture-hall, a common wooden tray being alternately held on the top of the rod and removed again. The rod itself had not surface enough to give vibrations which can be heard, but the larger surface of the tray gave the "magic music."
An ingenious and very simple method of measuring the velocity of sound in air and other gases is described by M. Bichat, in the "Journal de Physique." A tube about ten metres long, made of tin plate, is bent so that its extremities A and B are near together. The end A is closed by an India-rubber membrane; the end B carries a cork with a glass tube through it, which communicates, by means of an India-rubber tube, with a Marey's manometric capsule. These capsules are arranged in front of a blackened cylinder, so that the extremities of their levers rest upon the same generating line. Close by these a tuning-fork, making one hundred vibrations per second, is placed, and inscribes its vibrations side by side with those of the manometric capsules. The experiment being so arranged, a slight shock is given by the hand to the membrane A, the blackened cylinder meantime being turned. The capsules register the point of departure and the point of arrival, while the tuning-fork gives the time. In this way the velocity of sound in air was found by M. Bichat to be 333.3 metres per second. By means of two tin tubes, placed one above the other, we may in a single experiment demonstrate the difference of velocities of sound in air and in hydrogen; but it is difficult, in consequence of diffusion through the India-rubber, to keep the tube full of pure hydrogen.
Recent Exploration of Wyandotte Cave.—Wyandotte Cave, in Crawford County, Indiana, has a total length of twenty-three miles, including all the avenues; it includes many fine halls and domed chambers, the largest of which has a circumference of one thousand feet, and is said to be two hundred and five feet high. The Rev. H. C. Hovey mentions, in "The American Journal of Science," an important discovery made in this cave last April by a party of students from Wabash College. Forcing their way through a low, narrow passage from the locality known as Rugged Pass, the party entered a realm of chaos. "Pits, miry banks, huge rocks, are overhung by galleries of creamy stalactite, vermicular tubes intertwined, frozen cataracts, and all, in short, that Nature could do in her wildest and most fantastic mood." One of the curiosities of this place is a row of stalactites on which a musical chord can be struck or a melody played. What is known as the "Old Cave" was worked by saltpeter miners in 1812, and sundry acts of vandalism have been charged on them which more probably were done by the aborigines. The finest stalacto-stalagmitic column probably in the world is the Pillar of the Constitution in this "Old Cave." It is forty feet high, twenty-five feet in diameter, and it rests on a base three hundred feet in circumference. The weight of this immense mass of alabaster caused the underlying rocks to settle, and this in turn cracked the base, caus-
