.1863.] 



Trains passing through a Tunnel. 



69 



work around me. Similar shots were fired when the engine was at the centre 

 of the 4th shaft (which could be seen from above), ^he times of these sig- 

 nals were taken by an assistant. During this time I was at the telescope, 

 and noticed the second when any peculiar phase of disturbance appeared. 



The computation of the distance of the engine from the eye at a given time 

 is very simple. From the known distance of the south end of the tunnel and 

 the 4th shaft from the eye, we know the times taken by the sound of the gun 

 to reach the observatory. The temperature was during the whole series so 

 near 32° that the velocity of sound for that temperature, 363*13 yards, may 

 be used without sensible error. The effect of wind must also have been 

 insensible. Hence the signal from the south entrance was P*77 too late, 

 that from the shaft P'84. 



Correcting the times and dividing by their difference the distance of the 

 shaft from the entrance, 1162 yards, we have the velocity of the train (which, 

 however, I have given in miles per hour, as affording a more familiar 

 measure of the disturbing power). Then the difference of the time of 

 phase and corrected time of entrance gives the place of the engine on the 

 line, and the perpendicular is given. 



In the following record of the observations, the first column contains the 

 number, the second the times, the third the facts observed, and the fourth 

 gives the distance, then follow occasional remarks. In the disturbances, 

 I specially recorded as most definite the cross (4), and the arrangement 

 of bars of parallel stars (5). The slighter disturbances which precede or 

 follow the former were seldom entered, though quite sensible. 



1847, February 24.— I. 



No. 



Time. 



Observations. 



Yards. 



Remarks. 





h m s 









I 



7 i8 43 





845 



Velocity 11 -oo, miles an hour; weight 





7 19 21 



Shaft gun. 





of train 77*5 tons ; twelve carriages. 





7 22 57 



South gun. 







2 



7 23 8 



Lost sight of cross 



704 





II. 



3 



7 34 





34 8 





36 31 



4 



36 48 



Cross 



Shaft gun. 

 South gun. 

 Lost sight of cross. 



599 



780 



Velocity i6-6 miles; train 69-5 tons, 

 231 feet long; ten carriages. Ther- 

 mometer 24°. 



III. 



5 

 6 



7 44 40 jCross ; star very faint... 

 44 44 Shaft gun. 

 47 38 i South gun. 



680 



Velocity 13-8 miles. Star invisible to 

 the naked eye. Train 58*5 tons; 

 engine 14-5 tons; length 185 feet. 





678 





