236 



which a very powerful colHmating eyepiece has been applied, and, 

 having the telescope pointed to the mercury trough, and the reflect- 

 ed wires in view, I noted carefully the times and the characters of 

 any defalcations from good definition. Meanwhile the assistant 

 astx'onomer had gone to the part of the railroad nearest to the Ob- 

 servatory with a chronometer, and noted the times of any trains 

 passing, their speed, and the number of carriages. On his return, 

 the lists of times being compared, it was found that no result had 

 attended long trains moving slowly or short ones moving quickly, 

 but that the long trains moving quickly had produced a barely sen- 

 sible effect in spoiling somewhat of the definition of the reflected wires. 

 Never had this disturbance, however, amounted to a quantity that 

 need have prevented an observation being taken. In a word, the 

 disturbance was practically quite unimportant, and this, with an 

 apparatus so sensitive that a slight tapping with the hand on the 

 great stone-pier, containing about 120 cubic feet, produced so great 

 an effect as to render the wires for a time altogether invisible. 



Moreover, by comparing the amount of railway vibration observed 

 here, with that found at Greenwich and other observatories on loose 

 and soft soils, we find it to be less than a third of what is experienced 

 there. This result, so contrary to the usual belief of the facility 

 with which rock conducts vibration, is perhaps attributable to the 

 circumstance, that whatever vibration is produced in the hard, un- 

 yielding material, is very small, while that in the softer, looser soil, 

 is very great and violent at the place. In the rock, the wave, such 

 as it is, may travel quicker and farther, and with the characteristics 

 of a high musical note, than one of the same initial size in gravel ; 

 but the wave produced in the gravel by the same disturbing cause 

 appears to be so much larger at the place, as to be able to travel to a 

 very great distance, though with a slower motion and a lower note 

 (if any be audible) than in rock, and to be felt to a greater extent 

 within a certain range. 



The whole result is thus highly satisfactory for the stability of 

 the Edinburgh instruments ; since we have not only, by reason of 

 this rock foundation, an immunity from the prejudicial action of 

 water penetrating the ground and heaving up the piers, but there is 

 also such a decided lessening in the amount of vibration, and the 

 disturbance of any optical image seen in the mercury. 



3. On a General Method of efiecting the substitution of 



