278 REPORT — 1851. 



with 12 cells in action, a solid copper wire of nearly a quarter of an inch in 

 diameter could be almost instantly soldered on to a shilling laid on one of the 

 binding screws of the opposite terminal plate. The exciting fluids, as actually 

 used, will be stated hereafter. 



Canister complete. 



It will now be convenient to describe the instrument which I had con- 

 structed for the purpose of rendering visible to the eye the very feeblest 

 impulses or shocks, when communicated from the shaken ground to the in- 

 strument. 



The original idea of this instrument, which I have named the Seismoscope, 

 and to whose sensibility much of the success of these experiments is due, 

 suggested itself to me from observing the facility with which the image of a 

 fixed star was obliterated when reflected from the surface of still water by 

 the slightest disturbance of the fluid ; and I at first proposed to observe through 

 a telescope the image of the flame of a candle reflected at an incident angle of 

 45° from a trough of mercury, and take the moment of its disturbance from 

 the disappearance of the flame. On communicating this however to my friend 

 Dr. Robinson, Astronomer Royal, Armagh, who has throughout taken a 

 lively interest in these experiments, and to whom I am indebted for many 

 valuable suggestions and aid in the discussion of our results, he proposed 

 to substitute for the candle-flame the image of a pair of cross wires placed in 

 the focus of an achromatic object-glass, incident at 45° on the mercury, and 

 received by another achromatic telescope at an equal angle, provided with 

 one or two crossed wires intersecting the image of the former, and to note the 

 total disappearance of the former at the moment of impulse. 



The importance of this improvement was evident, and the instrument was 

 constructed accordingly. 



The Seismoscope (PI. XIII. fig. 3) consists of a cast-iron base plate, a a, 

 of 20 inches in length and 8 inches in width, and ^ an inch in thickness ; on 

 the centre of the surface of which is placed a cast-iron rectangular trough, b, 

 of 12 inches long, 4 inches wide, and 2 inches in depth, accurately planed 

 and brought true in all its surfaces, and black varnished externally. At 

 either end of this trough a brass vertical sliding pillar is screwed in, one of 

 which carries a tube, c, provided with an achromatic object-glass at its lower 



