66 Sir J. Soutli — Vibrations caused by Railway [1863. 



The tunnel runs under the park of the Earl of Essex ; and though I had 

 not the honour of a personal acquaintance with the Noble Earl, nor any 

 introduction to him, yet on learning my objects he transmitted to me by 

 return of post, from Carlsbad, a carte-blanche to erect my observatory 

 wherever I pleased, though it were in the very heart of his choicest game 

 preserves. To him therefore is mainly due whatever benefit may accrue to 

 science or to the Royal Observatory from the experiments recorded in this 

 communication. 



The point I selected was 302 yards distant from the centre of the line ; 

 and the perpendicular from it on the axis of the tunnel meets that at a 

 point 567 yards from the southern or London end of the tunnel, 1245 

 yards from the Triug or north end, and 594*5 from the fourth shaft. This is 

 the centre of the Observatory which I erected there : it is of wood, as small 

 as is consistent with the necessary accommodation, both for portability and 

 that it might be less agitated by the wind. 



It is quadrangular, 12 feet by 10, and its length is in the meridian ; the 

 eaves are 8*5 feet, and the ridge of the roof 10 feet above the floor, this 

 last being 4 inches above the ground, which is nearly level with that over the 

 tunnel. The roof is covered with tarpaulins very well secured, so as not 

 to be torn by a gale of wind. In the south and west sides are four windows, 

 which can be opened or shut at pleasure, to hght the Observatory by day, or 

 to see powder or other signals at night. In the roof is no opening ; but in its 

 northern side there is one which can be shut as required : it is little larger 

 than what is absolutely necessary to allow the reflected rays from the Pole-star 

 to pass uninterruptedly to the observer's eye through its whole revolution. 



At its centre, parallel with its sides and resting on the undisturbed gravel 

 4 feet below the surface, is a mass of brickwork laid in excellent Roman 

 cement, 8 by 3-5 feet at bottom, 7 by 3*5 at top, its length running east and 

 "west. On this stand two piers of similar brickwork, 1 8 inches by 14, and 46 

 inches higher than the floor : they are capped by two Portland stones of 

 similar horizontal section 8 inches thick. In the interior faces of these 

 stones are firmly fixed the Y-plates, which carry the Ys on which the in- 

 strument's pivots rest. 



Eighteen inches north of the brick massive, but in the same plane with 

 its base, is the centre of the hase of another pier, brought up also in Roman 

 cement, 24 inches from N. to S., 18 from E. to W. ; and it rises 12 inches 

 above the floor. The upper surface is perfectly horizontal, and serves to 

 support a vessel which contains mercury. Both this pier and the massives 

 are insulated from the floor, and touch the ground only at their bases. 

 The mercury-vessel was 18 inches by 4|, with its length in the meridian. 



The transit-instrument of the Campden Hill Observatory is far too pre- 

 cious to be exposed to the risks of such an expedition ; I therefore had one 

 constructed which might be considered an excellent substitute. The object- 

 glass (which under favourable circumstances will bear a power of 1000) is 

 87 inches focus and 4*75 aperture. The transverse axis is 31 inches ; and 



