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Sir J. South — Vibrations caused hy Railway [1863. 



The engine is not so paramount a disturber as might be expected, — the 

 heaviest, and even a pair of them, not causing more tremor than occurs 

 with the smaller ones. 



In taking the cross of stars as the test of disturbance, I must observe that 

 I do so, not because it is the earliest v^hich appears, but because it marks 

 distinctly an agitation greater than what is likely to occur at an observa- 

 tory subject to ordinary perturbations. These produce in such a mercury- 

 vessel as I used a single line of stars perpendicular to the length of the 

 vessel. It should seem that then only one set of undulations fit to produce 

 these images is excited in the mercury, the direction of which is regulated by 

 the sides of the vessel *. The existence of the cross shows that a second set 

 of waves perpendicular to the first has been developed : this always happens 

 if the sides of the vessel are equal ; and its occurring when they are so un- 

 equal as in the present case seems to indicate a corresponding excess of the 

 power which causes them. If the agitation be still greater, it seems as if each 

 of the images which form the cross became the origin of a row of secondary 

 images, the result of which is the form (5), a series of parallel rows of stars 

 varying from two to ten, or even filling the whole field. This token of ultra 

 disturbance is confined between lines making angles of 45° with the per- 

 pendicular to the rails — in other words, to distances under 427 yards, and 

 when the train is nearly in the centre of the tunnel. It is (except in two 

 instances) only seen when the cross is visible beyond 1000 yards : when the 

 agitation is still further increased the images vibrate in every direction, and 

 with yet more of it the whole becomes a mass of nebulous light ; of both 

 which some examples may be found in these observations. 



The opinion maintained by the late Mr. Robert Stephenson, that much 

 of these railway tremors were due to the sound of the train, although not 

 probable, induced me to try some experiments by firing cannon, maroons, 

 and rockets at various distances. 



One of these cannons (for I had two, each f of a pound calibre) heavily 

 loaded, at 300 yards produced (5), cross, and line simultaneously with my 

 hearing the reports ; but all disturbance was over in about \'b second. At 

 2020 yards there was the cross synchronous with the report, and of the 

 same momentary character ; and even at 3000 yards the cross could be 

 traced. This seems to have been due to the momentary impulse of the 

 sound-wave, for the continuous roar of two-pound rockets fired at 82 feet 

 from the mercury, though very lou*^, disturbed it very little ; while the ex- 

 plosions of eight ounces of powder iii their heads about 800 yards above the 

 ground produced all, the (5 j, cross, and line. A still more interesting expe- 

 riment was, firing the cannon in the tunnd at the point where the perpen- 

 dicular from the observatory met it. In t^Svcase two disturbances were 

 seen — one propagated through the ground, tn^ther through the air with 

 about a second of time interval. The sound probably made its way chiefly 



* For details on this I may refer to my Report to the Admiralty, published by order of 

 the House of Commons, July 6, 1846. 



