234 



With gentlemen who will adhere to a favourite theory of their 

 youth, in spite of all the myriads of facts contained in the volumes 

 since published by the Observatory, I fear that the few additional ones 

 which could be condensed into this paper would make but small im- 

 pression. Some very peculiar instances, however, can now be brought 

 forward; for there are at present in the neighbourhood far more power- 

 ful shaking influences than in those former days, and the existing in- 

 strumental means are more sensitive than ever to detect vibrations. 



These increased means of shaking are the introduction of railways 

 into the vicinity of the Observatory, and the running along them 

 at high velocities of long and heavy trains, creating a far greater 

 disturbance in the soil than the rumbling of any number of carriages 

 along Waterloo Place. 



The improved method of detecting the effect of this disturbance 

 is the recent adaptation of the collimating eyepiece, with modifica- 

 tions allowing of unusually high magnifying power and with good 

 definition, and its employment in combination with a trough of fluid 

 mercury. 



Tested in this way, a vibration is undoubtedly perceived at times, 

 and nowhere could we expect to be entirely free ; for such a univer- 

 sal cause as the wind striking on the outside of a building would 

 produce some degree of tremor in the subjacent soil. But the ques- 

 tion here is, Does the vibration take place to such an extent as to 

 vitiate the observations ? 



In answer to this I say, Certainly not ; for during the last five 

 years, the collimating apparatus has been in weekly, if not in daily, 

 use with the transit instrument, and on no single occasion was there 

 ever any impediment to accuracy of measure caused by vibration 

 transmitted through the ground from any of the neighbouring roads 

 or railroads ; — though from the remarkable sensibility of the appa- 

 ratus employed, the effects of the wind shaking the building, or per- 

 sons walking about, in, and even immediately around it — circum- 

 stances not peculiar to the Calton Hill — have sometimes impeded the 

 observations. 



But inasmuch as on each day that the observations were made, they 

 lasted only about twenty minutes, the theoretical shakers may possibly 

 suggest, that chance had always hit on the times of no railway trains 

 being on the move. Recently, therefore, I made a more crucial 

 experiment, and in this manner : — 



I stationed myself one day for three hours at the mural circle, to 



