the surface, and others at greater depths, where more dense and elastic strata produced changes in the direction of the wave front and yielded higher velocities. The whole subject is evidently too complex to warrant definite conclusions as to the velocity of ordinary earthquake waves, where the intensity of the original disturbance must always remain unknown. The results derived by Captain Knight from his observations at Astoria are in accordance with this view. (See C.) THE PHOTOGRAPHS. Referring now to, the photographs (see Appendix E for details), the first two views show that ignition by sympathy was transmitted so rapidly as to be practically instantaneous over the entire area of the rock, and that no failure occurred in any of the galleries. In a word, they demonstrate the entire success of this new method of ignition. The following computation fixes the epochs of these pictures, and of the explosion itself as well, referred to the chronograph record at Astoria. The first motion of the key is taken as the zero of time: Hence the explosion occurred 0.03 of a second before the record—or practically at the same instant. The register record shows that Lieutenant Lusk's first attempt (which failed to give a picture) was made 0.17 of a second after the automatic picture, and that his successful photograph was taken 0.25 of a second still later. Hence the actual epochs of these pictures after the explosion, closely determined, are 0.2 and 0.6 of a second, respectively. The epoch of Sergeant von Sothen's picture, roughly deduced from the increase in height of the jet as compared with those of the other two, is about two seconds. The extreme height was reached probably about a second later. An interesting result was obtained, accidentally, by the method used for recording these times. The actual chronograph record was as follows: First indent (recording automatic photograph), a smooth line .13 of a second long; interval, .06 of a second: second indent, three dots covering .11 of a second; interval of .12 of a second; third indent, three dots covering .13 of a second. The length of the second's mark on the fillet was about 1 inch, rendering these small periods easily measurable. They indicate that the first picture was taken at or before the arrival of the earth tremor, and that the others were taken during its passage. The distance to the rock being about 1,100 feet, this would suggest that the wave was moving with a velocity of about 1 mile per second, or much less than that with which it reached all the stations beyond Willets Point. But little weight can be accorded to this inference, because the wave velocity is much too high, and the instant of explosion too uncertain, to permit a precise determination of so small a difference of time; still the result, so far as it goes, is confirmatory of the Willets Point record, and of a rate of horizontal advance in the vicinity of the mine much more sluggish than at greater distances. Very respectfully, your obedient servant, HENRY L. ABBOT, |