ON THE FACTS OF EARTHQUAKE PHiGNOMENA. 279 



end, and a pair of cross wires at its focus. The other carries a small achro> 

 matic telescope, d, of the same aperture and provided with one vertical wire. 

 Both tube and telescope are jointed so as to admit of motion in the vertical 

 angle, and by means of the sliding pillars and screw-collars on them, can 

 either be rotated round the axis of the pillars or moved up and down parallel 

 therewith. 



When the cast-iron trough is filled to the depth of 1 inch with clean 

 mercury, and the perforated blackened tin cover, e, e, shown in dotted lines, 

 laid on it, which excludes all light except what can reach the surface of mer- 

 cury through the tube or telescope, and when the latter are placed in the 

 same plane, and at angles of 45° to the surface of the mercury =90° to each 

 other ; then, while the cast-iron base plate and trough are perfectly still, the 

 image of the cross wires of the tube are seen steadily reflected in the surface 

 of the mercurial mirror, and intersected by the vertical wire of the telescope ; 

 but the slightest movement communicated to the whole instrument, or to 

 any part of it, causes this image either to flicker or to disappear wholly for 

 the moment. 



In daylight the image is produced of course by the light of the sun ; but 

 as it was thought possible that these experiments would require to be con- 

 ducted during the silence and quiet of night, a lamp, k, sliding upon a move- 

 able iron rod, was added ; parallel rays, transmitted through a screen of oiled 

 parchment from this lamp, were adapted to pass into the tube, c. This part 

 of the arrangement also was suggested by Dr. Robinson, but we never found 

 night observations requisite. 



As the movement or disappearance of the image is caused by the momen- 

 tary passage of a small wave across the face of the otherwise undisturbed 

 mercury, and as the front and rear slopes of this wave in their passage across 

 the field act as the surfaces of a mirror whose angles of inclination to the 

 line of collimation of the cross-wire tube are variable, so, the angles of inci- 

 dence and reflexion being always equal, any deviation produced is doubled 

 when seen in the axis of the collimating telescope. 



This telescope has a focal length of 740 inches. If F=this focal length, 

 V=the distance of distinct vision=(for R. Mallet) 12 inches, and M=the 

 magnifying power of the telescope, then 



Mx — =the magnifying of an object seen in the apparatus, 

 F 



or 



7 xii=|i=ll-39. 



7-4 7-4 



Hence the total power of the instrument to magnify any small disturbance 

 and make it apparent is 2x ll"39=i22*78, or nearly 23 times. 



Its actual sensibility is very great, such, that resting upon solid granite 

 rock, a blow of a light hammer on the rock can be perceived at 100 yards 

 off; a stamp with the foot at 50 or 60 yards, and on compact sand or clay, a 

 horse trotting can be observed at a quarter of a mile away. In towns the 

 motion of the most solid buildings or of the ground is so continual that it 

 is not possible anywhere to see the cross wires at all. 



As the transit of any elastic wave or pulse is rendered evident by this in- 

 strument, in virtue of the production and transit of a small fluid wave through 

 the mercury of the trough and along its surface (a small wave of translation), 

 and as this wave is observed at the moment of its transit across the middle 

 point of the trough, which is intersected by the optic axes of the two tele- 

 scopes, and as the time occupifd by the raising and transmitting of this 



