360 Mr. R. Mallet on the Rate of 



printed in the British-Association Reports for 1851, and ac- 

 companied by an engraving (plate 13) of the instrument. 

 The circumstances are the more extraordinary, because my seis- 

 moscope and its magnifying-power are also actually described 

 in the account of my experiments made in the rocks of Holy- 

 head, printed in the ' Philosophical Transactions ' for 1861, with 

 which General Abbot seems to have been acquainted. He 

 will there find also, at page 279, that the magnifying-power 

 of my instrument is 22*78, or nearly 23 times; and this de- 

 gree of magnifying-power I have found sufficient under all 

 circumstances. I may notice, however, that in some experi- 

 ments made a few years since by me, at the desire of Sir G. 

 B. Airy, Astronomer Royal, of Greenwich, in concert with the 

 then chief assistant of that observatory, Mr. E. J. Stone, now 

 Astronomer at the Cape of Good Hope, and with Mr. Car- 

 penter, one of the Greenwich computers, a seismoscope iden- 

 tical in principle with my own, but of much greater magnifying- 

 power, was employed, the power being capable of modification 

 by changing eyepieces. Any determination of wave-transit 

 velocities was beside our object; but this was remarked di- 

 stinctly, that changing the magnifying-power of the in- 

 strument produced no noticeable change in transit-velocity as 

 indicated by it ; nor was any such, I believe, noticed by the 

 late Sir James South in his experiments, made many years 

 ago, to determine the extreme radius of the area caused to 

 vibrate by railway trains passing through Kilsby tunnel. In 

 all these instances, however (except possibly that of Sir James 

 South, as to which I possess no details), the seismoscope was 

 used in the only way by which it can afford trustworthy and 

 comparable results as to the instant of transit of the seismic 

 wave as seen in the instrument, namely by bringing the hori- 

 zontal wires of the illuminating achromatic object-glass parallel 

 and near to the horizontal wire of the observing-telescope, and 

 always noting and adopting as the instant of wave-transit the 

 instant at which the image of both these wires became rapidly 

 blurred or confused and suddenly invisible. This method, 

 which does not seem to have been adopted in any of the Hal- 

 let's-Point observations, is greatly to be preferred over any 

 supposed observation of the earliest access to view of the front 

 slope of the advancing wave, which, in reference to time, must 

 always be a matter of great uncertainty. As to the duration 

 of the vibratory disturbance in the field of view of the instru- 

 ment, to which importance seems to have been attached in the 

 Hallet's-Point experiments, it is quite delusive as affording 

 any precise or useful information as to the dimensions or time 

 occupied in the transit of the earth-wave or wave of shock 



