1863.] Sir J. South — Vibrations caused by Railway Trains, 65 



Experiments^ made at Watford^ on the Vibrations occasioned by- 

 Railway Trains passing through a Tunnel/^ By Sir James 

 South, LL.D., F.R.S., &c., one of the Visitors of the Royal Ob- 

 servatory of Greenwich. Received June 17, 1863*. 



In the year 1846 an attempt was made to obtain the consent of the 

 Lords of the Admiralty to run a railway through Greenwich Park, distant 

 only 860 feet from the Royal Observatory, which would in the opinion of 

 many competent judges have been most injurious to that Establishment. 

 Such consent their Lordships refused ; but as I was assured on high au- 

 thority that this attempt was to be repeated, and that too with the fullest 

 confidence of success on the part of its projectors and supporters, I deter- 

 mined to make experiments which might bear more decisively on the 

 question of railway tremors, as affecting that Observatory, than those pre- 

 viously made by myself and others. 



For this purpose it seemed indispensable that the station selected for 

 making them should geologically resemble that of Greenwich, and that 

 the astronomical means employed to detect the existence and determine 

 the intensity of the tremors should be, optically, at least equal to the te- 

 lescope of the Greenwich Mural Circle. 



As much importance , was attributed by the advocates of this railway to 

 the supposed power of a tunnel to render the vibrations imperceptible, it was 

 also desirable that it should be one of the conditions of these trials. 



Having but little more than a popular knowledge of geology, I relied on 

 my old and valued friend the late Mr. Warburton, who had recently been 

 President of the Geological Society, to guide me in the choice of a station ; 

 and it was on his authority that I fixed on the Watford Tunnel and its im- 

 mediate vicinity. 



There, under a light gravelly soil of 18 or 20 inches deep, lies a bed of 

 gravel of considerable but variable thickness, sometimes compact, at other 

 times loose, and immediately under it chalk with occasional flints. 



The tunnel, of which the bearing is 41° 19' to N.W. of the meridian, 

 and by my measurement is 1812 yards long, passes principally through 

 chalk ; its arch is about 24 feet in diameter, the crown of it being about 

 21 '5 feet above the rails. The thickness of the brickwork is about 18 

 inches; the mean thickness of the chalk above the crown of the arch 

 about 50 feet, whilst that of the gravel, though subject to great irre- 

 gularity, may perhaps be regarded as 14 feet. If so, we have outside the 

 tunnel above the horizontal plane of the rails 87 feet of chalk, flint, gravel 

 and soil, constituting an assemblage of which the power of transmitting 

 tremors must be comparatively feeble. 



There are five shafts in the tunnel, four of which are circular, 8*5 feet 

 diameter, and one quadrangular, about 26 feet by 34. 



* Read June 18, 1863 : see Abstract, vol. xii. p. 630. 



