EARTH TREMORS. 307 



Captain Kater found that he could not perform his 

 pendulum experiments in London on account of the 

 vibrations produced by the rolling of carriages. Captain 

 Denman, who made some observations on artificially 

 produced tremors, found that a goods train produced an 

 effect 1,100 feet distant in marshy ground over sand- 

 stone. Vertically, however, above a tunnel through the 

 sandstone, the effects only extended 100 feet. 



A remarkable example of the trouble which artificially 

 produced earth vibrations have occasioned those who 

 make astronomical observations occurred some twenty 

 years ago at the Greenwich Observatory. When deter- 

 mining the collimation error of the transit circle by 

 means of the reflexion of a star in a tray of mercury, it was 

 found that on certain nights the surface of the mercury 

 was in such a state of trembling that the observers were 

 unable to complete their observations until long after 

 midnight. After obtaining a series of dates on which 

 these disturbances occurred, it was found that they coin- 

 cided with public and bank holidays, on which days crowds 

 of the poorer classes of London flocked to Grreenwich 

 Park, and there amused themselves with running and 

 rolling down the hill on which the observatory is situated. 

 On these occasions it was found that the disturbances 

 in the mercury were such that observations could not be 

 made until two or three hours after the crowds had been 

 turned out of the neighbouring park.' 



To obviate this difficulty Sir George Airy suspended 

 his dish of mercury in a system of indiarubber bands, and 

 in this way succeeded in eating the intruders up. 



Lieutenant-Colonel H. S. Palmer, E.E., when engaged 

 with the transit of Venus expedition in New Zealand, in 

 1874, was troubled with vibrations produced from a 

 Palmer, Trans. Sets. Soc. of Jaiyan^ vol. iii. p. 148. 



