ON EARTH TREMORS. 151 



h. m. s. 



April 27. Decimation 7 57 13 p.m. 



Horizontal intensity . . . . 7 56 3-t „ 

 Vertical intensity . . . . 7 56 10 „ 



Kew (Mr. C. Chree), 51° 38' 6" N., 0° 18 47" W. ' There is a very 

 small but unmistakable movement in the horizontal force curve, and a 

 simultaneous extremely slight suggestion of a movement in the declina- 

 tion curve. Careful measurements give 8h. Om. p.m. as the mean Green- 

 wich time of the middle of the movement on the horizontal force curve, 

 and 7h. 59m. p.m. as that for the declination curve. There is not the 

 faintest trace of movement in the vertical force curve.' 



Birmingham, 52° 28' N., 1° 54' W. The pulsations were first seen on 

 April 27 at 7h. 59m. p.m. Between 8h. Im. and 8h. 3m. 20s. the image passed 

 the cross wire twenty times, giving an average duration of 14 seconds for 

 each oscillation. Between 8h. 8m. and 8h. 10m. 2s. the same number of 

 oscillations was completed, the average duration of each being 12-2 seconds. 

 The amplitude was determined by adjusting the image of the disc of light 

 so that at one limit of its movement its edge coincided with the cross- 

 wire of the telescope. At 7h. 59m. the range was equal to three-quarters 

 of the diameter of the disc. The whole diameter, it was afterwards found, 

 is equivalent to 0-98 inch of the scale, so that the trace of the disturbance 

 on a photographic recording apparatus in the same position as the scale 

 would have been 18 mm. in breadth. As the angular value of the scale- 

 divisions had not been ascertained since the beginning of August 1893 a 

 new determination was made on the evenings of May 16-18. The mean 

 of twenty-four pairs of tilts of 2" is 6-66 ±-08 inches of the scale. Thus, 

 at 7h. 59m. the range was 0"-22. After this I believe it slightly increased 

 until 8h. 2m. or 8h. 3m. At 8h. 5m. it was 0"-16. It then rapidly and 

 almost continually diminished, being 0"-ll at 8h. S^m., 0"-08 at 8h. 7m., 

 0"-05 at 8h. 8m., and 0"-03 at 8h. 12m. The movement then became so 

 small that it could only be estimated. It was about 0"-01 at 8h. 14m., 

 0"-005 at 8h. 16m. At 8h. 17m. there was a single oscillation of 0"-015. 

 From 8h. 18m. to 8h. 19m. the image was steady, but at the latter time 

 the range suddenly increased to 0"-03, but diminished after a few oscilla- 

 tions, until at 8h. 28m. the image was steady again. After this time no 

 movement so great as 0"-003 could with any certainty be detected.^ 



In addition to the above records it should be stated that the magnetic 

 curves on April 27 have been examined at the following observatories 

 with a negative result : Coimbra, Greenwich, Lisbon, Madrid, Nantes, 

 Nice, and Stonyhurst. 



The difference between the distances of Athens and Wilhelmshaven 



from Atalante is 1,910 km., and the difference between the recorded times 



at the same places is about 12m. 55s. on April 20, and about 10m. 9s. on 



April 27. Assuming that these times correspond to the same phase of 



the disturbance, we obtain 2-46 and 3-14 km. per second respectively for 



the average velocities on these days. These give : - — 



Time at epicentrum on April 20 = 5h. IGm. 25s. p.m., G.M.T. 



„ 27 = 7h. 45m. 40s. P.M. „ 



Using these values of the initial time, we have the following table : — 



' Owing to the lag of the mirror through the oil these estimates are probably 

 less than the actual amounts. When the frame of the pendulum is tilted suddenly 

 through an angle of 2", the image at first moves quickly, but during the first 

 15 seconds not more than half its total deflection is accomplished. 



^ Since the duration of the disturbance at Athens on April 27 was only 12 seconds, 



