VULCANOLOGY AND SEISMOLOGY. 485 



impulse in a normal direction, and similarly a bracket seismograph ar- 

 ranged to indicate normal motion begins its indications before a similar 

 instrument indicating transverse motion, implying that the normal 

 wave travels more rapidly than the transverse. Near to the origin the 

 normal motion is first outwards, then inwards, and the motion inwards 

 is greater and more rapid than the motion outwards ; while at a dis- 

 tance from the origin the first motion may be inwards, and the two 

 phases are practically of equal amplitude. Eoughly speaking, the am- 

 plitude of normal motion is inversely as the distance from the origin. 

 The laws of transverse motion are practically the same as those of nor- 

 mal motion, but less pronounced. IliTear to the origin the amplitude of 

 the transverse motion is less, but the period greater than that of the 

 normal motion. The velocity of transmission varies from 200 to GOO feet, 

 which is much less than the velocities obtained by Mallet and Abbot. 



On October 10, 1885, Flood Rock in Hell Gate was blown up by dyna- 

 mite. The preparations for this event had been going on for ten years, 

 and involved the excavation of galleries beneath the rock and the storing 

 therein of nearly 300,000 pounds of explosives. It was expected that 

 such an explosion would cause vibrations of the earth similar to an earth- 

 quake, and as the day for its occurrence approached preparations for 

 studying the propagation of this vibration were made both by the offi- 

 cers of the Engineers, under whose direction the work was done, and by 

 volunteers from the Seismological Conference in Washington, who went 

 to Xew York for the purpose, and established their instruments at such 

 points in the neighborhood as were found available. Directors of astro- 

 nomical observatories near New York were also requested by them to 

 co-operate by watching for the arrival of the earth wave. (Science, vi: 

 315.) Unfortunately the firing of the mine was slightly delayed, and 

 did not take place until about fourteen minutes after the hour announced. 

 This delay interfered seriously with the observations of the volunteer 

 observers, a considerable number of whom gave up the watch before the 

 explosion occurred, either supposing that the vibration had failed to 

 reach them or being deceived by some local tremor. It was, however, 

 caught at Princeton, N. J., and at Harvard College. The observations 

 obtained by the volunteers were quite discordant, but indicate a much 

 higher velocity than was obtained by Mallet, or by General Abbot from 

 the Hallett's Point explosion in 1876. General Abbot had arranged a 

 series of observers to Patchogue, L. I,, in one direction, and to West 

 Point in another, who were in telegraphic communication with the firing 

 I)oint at Astoria. {Science,Yi: 431.) His results, although also discord- 

 ant, give velocities much higher than those deduced from natural earth- 

 quakes. (Science, vii: 25.) 



An instrument for the automatic registration of earth-tremors and 

 eartli-tips is described by John Milne (Nature, xxxii : 259). Its essen- 

 tial parts are a i3endulum one meter long, suspended from a tripod about 

 5 feet high, and acting upon the short upper arm of a light balanced 



