216 



SCIENCE. 



[N. S. Vol. XI. No. 267. 



novel to be worthy of record. The eifect 

 was noticed here, upon setting the telescope 

 for the Nadir observation, on the night 

 of Friday, January 19th, the date of the 

 earthquake, most seriously felt at Colima, 

 Mexico. 



The reflected images of the threads were 

 at once seen to be swinging across the field 

 of vision, with a regular oscillation, much 

 like the swing of a pendulum. The period 

 was about seven seconds for the full ampli- 

 tude of the swing. The extent of the arc 

 could be exactly estimated, in the east and 

 west direction, by the fixed transit threads, 

 and appeared to be of the same amount in 

 the north and south direction. The reflec- 

 tion from the surface of mercury doubles 

 the actual angle of inclination, which was 

 very closely 15", for the full arc of vibra- 

 tion, at the maximum seen. 



This corresponds to a total displacement 

 of the eye end of the telescope of ^io of an 

 inch. A movement of twice this amount in 

 the surface of the earth, when produced 

 suddenly, will ordinarily be detected as a 

 slight earthquake shock, and will be re- 

 corded by delicate seismographs. It would 

 fall at II. of the Rossi- Forel scale, and is 

 about as slight a vibration as can usually be 

 detected. The delicacy of this test is shown 

 by the fact of the oscillations being still 

 easily perceptible when they had fallen to 

 less than one-tenth of the above maximum. 



It is probable that vibrations in the earth's 

 crust of much larger extent would still be 

 entirely imperceptible to the senses when 

 taking place as slowly as in the present 

 case. They are believed to extend to great 

 distances from the active center of the dis- 

 turbance, at least in special cases ; and 

 may perhaps precede the sensible shock by 

 a considerable interval. In the additional 

 instance, recorded below, there is little 

 doubt that the earl}' vibration was the fore- 

 runner of an earthquake shock, which was 

 local only, as far as known. 



In the present case, if the same vibrations 

 extended to the summit of our large dome, 

 they would amount to but the twentieth 

 of an inch in actual displacement ; and be- 

 ing of so slow a period, no injury would be^ 

 anticipated. We have records here of dis- 

 placements of the earth's crust during earth- 

 quake shocks of twenty times that of Janu- 

 ary 19th, coming as sharp and continued 

 vibrations. 



The oscillations were first noted at lOh. 

 50m. Pacific standard time, eight hours 

 slow of Greenwich. Local time would be 

 six minutes less. They continued of the 

 same amplitude, apparently, for five min- 

 utes at least, and at 11 h. 5m. had dimin- 

 ished to about one-quarter of the maximum. 

 At llh. 15m. they were still easily percep- 

 tible, but appeared no longer as uniform ; 

 the reflected image would appear stationary 

 for several seconds and then continue the 

 swing. 



At llh. 30m. the vibration had so nearly 

 ceased that the Nadir observation was 

 made, in order to determine whether any 

 change had taken place in the position of 

 the instrument. The comparisons with 

 preceding and following determinations in- 

 dicate no change in any particular. The 

 swing was nearly as regular as a pendulum 

 at the beginning, and there was little varia- 

 tion in successive swings, except as they 

 diminished in intensity, just as a pendulum 

 comes to rest. 



The night was at that time nearly calm ; 

 the wind, which had been earlier light from 

 the northwest, having died out. It should 

 be added that the Nadir observation is 

 usually easily made here. There are no 

 shocks to be anticipated of any character 

 at night, and the wind, even at 40 miles an 

 hour, does not interrupt the reflection of 

 the images. When coming in strong gusts 

 there is no vibration at the surface, either 

 coming from direct pressure or carried 

 down by the walls of the building. The 



