1863.] 



Trains passing through a Tunnel. 



81 



That these results may be more easily appreciated, I have condensed the 

 most important of them into the preceding Table, which gives in one view 

 the distance at which that amount of disturbance begins and ends which 

 produces the cross, that at which the far greater one occurs causing the 

 appearance (5) (a system of three or more parallel rows) wherever it does 

 appear, and the velocities and weights of the trains when known. 



It is evident from this Table that the tremor which is sufficient to 

 produce that disturbance of the mercury which shows a cross of stars is 

 propagated to considerable distances — in one case to 1 1 76 yards ; and 24 

 per cent, of the entire are above 1000. Such distances do not pass the 

 northern end of the tunnel, but go far beyond the southern. In the latter case 

 the vibrations are excited while the train is in an open cutting ; and those 

 who suppose that the tunnel has much power in deadening them would of 

 course expect that' they would be sensible at a greater distance than at the 

 other end. This does not seem, however, to be the case : and the Table 

 shows that in this respect there is very little difference, if we take into 

 account another cause of inequality, namely, that the tremor is manifested 

 further at the exit than at the entrance of the train. The column headed 

 "Exit " shows by s. that^the exit was at the South end, and the entrance 

 at the North. 



Now, when the observations are examined where both were noted, we find 

 that the limit of the cross is greater at the exit than at the entrance in 2P 

 out of 39, or 74 per cent, of s., and 12 out of 16, or 75 per cent, of n. The 

 reason of this, I suppose, is that the long-continued action of the train on 

 the rails tends to produce a greater and more prolonged undulation in the 

 mercury. 



But the equal percentage shows that there is really no protecting power 

 in the tunnel against the lateral propagation of tremors, whatever may be 

 the case immediately above the crown. 



In general one might expect trains to produce disturbance in proportion 

 to their speed and their weight. To a certain degree this is true ; but the 

 exceptions are sufficient to show that other influences must be taken into 

 consideration. Examples of high speed with comparatively small effect 

 are afforded by the observations on March 18, IT. ; 19, III. ; and 29, V. 



Others of the reverse conditions are given by February 24, I., II. ; 

 March 18, YI. ; and specially March 16, III., in which with a velocity of 

 only 11 "4 miles the cross was shown at 1110 yards. This it may be 

 remarked is a decisive proof that any plan of protecting an observatory by 

 slackening the speed of trains passing near it is entirely useless, even if it 

 could be enforced. 



It is probable that one cause of this high disturbing-power in slow trains 

 is that already referred to, the long-continued accumulation of vibration, 

 the quick ones passing beyond distance before the mercury has got into full 

 vibration, the others having full time to do their work though with less 

 intrinsic force. On this account also long trains are more disturbing than 

 short. 



