The Flood Rock Explosion.

While the average college man was well aware of the local commotion due from the College House explosion, the earthquake from the somewhat greater, though no more historic explosion at Hell Gate recently, was also perceptible at Cambridge.

It is a fact, however, that the disturbance following the "Flood Rock" explosion made itself felt through the delicate apparatus at the observatory, and was carefully observed by Prof. Rogers and his assistants. As our local experiments in explosiology are supposed to have brought into prominence this branch of science, a description of the apparatus used by Prof. Rogers, seems of interest. A cup of Mercury is placed either directly upon the earth or upon a stone shaft which rests on bed rock. Upon the surface of the Mercury is thrown the image of illuminated pin holes in a metal card; or, in some instruments, any other image is used such as the reflection of micrometer lines in a star. Any change in the surface of the mercury causes a blurring of this image, which is reflected from the mercury upon a screen. The reflection is observed by means of a telescope or microscope placed so as not to be easily jarred. The utmost delicacy is obtained by this arrangement, for if a sufficiently high magnifying power is used, a movement along the screen of the reflected spot or image a thousandth part of an inch is readily perceived.

It will be remembered that the blast was exploded Oct. 10 at 11.13 o'clock, and a few seconds over. Prof. Rogers first observed disturbances in the mercury surface at 11.17.30, and from that time disturbance waves came at nearly regular intervals of about 18 seconds, until the maximum was reached on the fourth or fifth wave. Then the impulses declined in force and died away about as they had come. In addition to the screen Prof. Rogers made use of direct observation of the mercury surface through a microscope, thus giving him the added advantage of judging the wave's length, etc. which a direct view of them would furnish.

There is another instrument for detecting disturbances of terra firma. called the "Seismiscope" which was used by the Geological survey, and by Prof. Young at Princeton for this same explosion.

It is formed by a looped wire terminating in an indicative point, upon which rests a heavy weight, the whole being suspended from a firm support. This, however, has not the advantage of such great delicacy as the mercury cup and is used in the more transient stations of the various surveys.