October 16, 1885.] 



SCIENCE. 



327 



the U. S. geological survey, together with repre- 

 sentatives from the naval observatory and the 

 signal service. Although the results of this work 

 cannot be definitely ascertained for at least some 

 weeks, the readers of Science will doubtless be 

 interested in a brief account of the methods of 

 doing it. 



Doubtless the most certain method of detectmg 

 sUght earth-tremors is by means of a vessel of 

 mercury, from the surface of which is reflected 

 hght from a series of pin-holes in an opaque sheet 

 of metal or card-board, the image of the holes 

 being observed and studied by means of a tel- 

 escope properly secured against accidental dis- 

 tiu:bances. The shghtest ripple in the mercury 

 produces a blurring of the image of the small pia- 

 holes. Of course in an astronomical observatory 

 the reflected image of a star or of illuminated 

 micrometer lines will be equivalent to the above 

 arrangement, and in some respects superior to it. 

 By iucreasiug the magnifying power, the sensitive- 

 ness of this method of observation may be in- 

 creased almost indefinitely, easily beiug carried 

 beyond what is desirable. A serious objection to 

 it, however, is that the disturbance makes no 

 record of itself, requiruig, therefore, constant 

 attention from the observer during the entire 

 period covering the possible arrival of the wave to 

 be detected. The great convenience of the method 

 and the ease with which it can be used at most 

 astronomical observatories, taken in connection 

 with its great sensitiveness, justified the sending of 

 ch'culars inviting co-operation to directors of ob- 

 servatories within the possible radius of appre- 

 ciable disturbance. These were issued a week 

 before the explosion, and, on the night previous to its 

 occurrence, telegrams were sent to those who had 

 indicated a disposition to co-operate, informing 

 them of the exact hour fi^ed for the event. 



The party proceeding from Washington, con- 

 sisting of Professor F. W. Clarke, Professor H. M. 

 Paul and the writer, in addition to being provided 

 with the necessary apparatus for mercury ob- 

 servations, carried three seismoscopes and one 

 chronograph, together with a number of chronom- 

 eters. 



The seismoscopes were of a form recently de- 

 signed in the physical laboratory of the U. S. 

 signal service, and were constructed by Mr. 

 Ktibel, the mechanician of the geological sur- 

 vey. They were hastily prepared for this expedi- 

 tion, and have not yet quite reached their final 

 form, but a brief description of one may be of 

 interest. 



A steel wire about two millimetres in diameter 

 is bent so as to form a loop at the end about eight 

 cm. in length and twelve mm. in breadth, the 



sides parallel and the ends round ; from one end a 

 long needle of the same wire projects, its length 

 being three or four times that of the loop, and its 

 point very sharp. A cylindrical iron weight, of 

 one or two kilos, has a hole drilled through it along 

 the axis, large enough to freely pass the looped end 

 of the needle, one part of this hole being of some- 

 what smaller diameter than the other. A small 

 circular steel plate, with opposite sides cut away, 

 fits the larger bore and rests on the shoulder of the 

 smaller. Through the centre of this plate a steel 

 screw passes downwards, the point of which rests 

 in a small cup made in the centre of the round end 

 of the loop, from which the needle projects. The 

 upper end of the loop reaches a short distance 

 through the cylindrical weight, and hangs upon a 

 small hook rigidly secured to the cast-iron sup- 

 port. When in position, the steel points, on which 

 the weight is supported, are sufficiently above the 

 centre of gravity to insure stability. It is easily 

 seen that this point is approximately that about 

 which instantaneous rotation of the needle takes 

 place when the system is slightly disturbed, and 

 that any motion of the sustaining hook is greatly 

 magnified at the point of the needle. Just below 

 the latter is a sort of lever-trigger, moving freely 

 about a horizontal axis, from the short end of 

 which projects in a vertical direction a very fine 

 wire two or three mm. in length. To the long 

 arm of the lever is fastened a platinum wire shaped 

 like an inverted U, vrith the legs resting in mer- 

 cury cups. The instrument is ' set ' by lifting the 

 long arm until the fine wire at the end of the short 

 arm rests on the point of the projecting needle, in 

 which position it remains with the platinum wire 

 out of the cups and the circuit broken. A 

 slight jar produces motion in the needle, releasing 

 the short wire and allowing the long arm of the 

 lever to drop and close the circuit. The instru- 

 ment may be made as sensitive as is desired by 

 diminishing the size of the needle point and of the 

 small wire which rests upon it. The advantage of 

 an instrument of this kind is obviously its auto- 

 matic operation. It can be joined up with a 

 chronograph and left to register its drop without 

 further attention. 



One of these seismoscopes was thus connected 

 with a chronograph in the basement of one of the 

 buildings belonging to the Emigrant's hospital on 

 Ward's Island. No better point could have been 

 found, as it was in full view of and almost the 

 nearest point on land to Flood Rock. The writer 

 must express his indebtedness to Dr. Marple, the 

 physician in charge, and to others connected with 

 the hospital, for the generous manner in which all 

 facilities at hand were placed at his disposal in the 

 arrangement of this station. A mercury observa- 



