1863.] 



Trains passing through a Tunnel, 



67 



the Y has sufficient azimuthal motion to enable me to follow the Pole-star 

 in its whole course, so that at any hour (if clear) I could have the reflected 

 image of the star in the mercurial vessel ready to testify against the tre- 

 mors caused by a train. 



Supported by timber passing into the ground, but unconnected with the 

 floor and convenient to a writing-desk which occupies the S.E. angle of the 

 building, stands a journeyman clock. It is set by my excellent gold pocket- 

 chronometer, Molyneux No. 963, and rarely deviates from that more than 

 one- or two-tenths of a second in three or four hours. The clock of the 

 Watford Station was compared with the chronometer, going and generally 

 returning, for the purpose of identifying particular trains. 



These details will, I hope, suffice to prove that every precaution was taken 

 to obtain accurate results, and that those which I did obtain may be fairly 

 considered as identical with what would have been found in a first-class 

 observatory under the same circumstances of locality and traffic. 



I was at my post to commence observations on December 22nd, 1846 ; 

 but that and the three following nights were starless. The 26th was fine, but, 

 owing to the irregularity of the trains, and the want of well-organized signals, 

 I could only satisfy myself that all was in^good working order, and that the 

 trains caused great disturbance. For thirteen following nights I was at my 

 post, but in vain ; all was dark, with the thermometer from 22° to 31°. 



On January 11th, 1847, it cleared, and I observed seven trains with 

 decisive results, being able to announce their presence before it was known 

 to my assistants, who were on the watch outside the observatory. 



The Pole-star's image as reflected from the mercurial surface, when 

 no train was near, appeared 



As a very small, perfectly steady disk, thus — . . . . . •©(!.) 



which as the train approached broke up into a quin- 1 ^ ^ , . 



tuple, thus — J ' (^0 



'^*> ■ 



As the disturbance increased, the form be- "j 



came linear at right angles to the length V ^» • «o@® @® ® @ © © • • (3.^ 

 of the mercury-vessel, thus — . . .J 



When the train was considerably advanced 1 ® 



iu the tunnel, a cross formed, thus- . / ' • •••®©®|©®o®»- • (4.) 



And when near the perpendicular from the) 



observatory, three parallel lines of disks ap- t SI|||nS§ni2§l (5.) 

 peared, thus — . . • I ©«®9eo©(s©ij@©©9®, ^ ^ 



