1897 - 98 .] Note on Indian Earthquake , 12 th June 1897- 15 
usual sensitiveness of the photographic paper used at this Obser- 
vatory has been ascertained by experiments with a steady light, and 
found that an exposure of about four seconds to the usual kerosene 
burners (of about four candle-powers intensity) used, is enough for 
action ; and with regard to impression of light during the vibra- 
tion of the magnet, it is found that oscillations of amplitude of 
division of the scale, equal to a displacement of about 28 
minutes of declination and upwards, are not recorded ; the velocity 
of the speck of light, together with the motion of the paper, a 
little over half-an-inch in one hour, precludes photographic action, 
unless the amplitudes are brought within the above limit ; and 
naturally the first impressions produced are at the extreme ends 
of the amplitude, where the velocity is a minimum. It is only 
when the vibrations fall to within about 1J divisions of scale that 
the paper shows the impression of light in the middle of the 
curves — that is, where the velocity is a maximum. 
It is assumed in what follows that every seismic wave was 
accompanied by a temporary disturbance of the magnetic circula- 
tion. The first wave, which appears to have just commenced the 
disturbance in the instrument , must have passed Colaba at about 
5 minutes past four. The seismic disturbance then seems to 
have grown in intensity, and about 7 minutes past four the 
increased amplitudes (which, by the way, it must be noted, were 
first performed about a higher zero showing a decreased easterly 
declination, movement of the curve in the downward direction 
showing an increase) passed the limit above referred to, and the 
trace is entirely lost. Immediately after this, however, the impul- 
sive force appears to have ceased, and the damper of the magneto- 
graph reducing the amplitudes [the logarithmic decrement has 
been ascertained to he = '035] brings them within the limit of 
photographic action, and the trace is just photographed at about 
nine minutes. This marks the time of temporary lull, for imme- 
diately afterwards the trace is lost, to appear faintly again, hut 
more strongly than before, at about eleven minutes. This goes on ; 
but the magnetic action now becomes less and less intense (presum- 
ably, therefore, the cause of the disturbance the seismic waves 
also) ; and the amplitudes of the vibrations falling within the limit 
of photographic action, the record runs beautifully clear after 
