Mr Stevenson on a New Signal-Light for Railways. 349 



power may fall upon the engineman's face, at so short a dis- 

 tance from the light itself, that it could not fail to be always 

 visible at that point, even in the thickest fog. 



According to the present practice, a comparatively feeble 

 light is exhibited at the station whose position it is intended 

 to point out, and this light, which is permitted to pierce the 

 gloom until its power is greatly diluted by the united effects 

 of its own divergence, and the length of its passage through a 

 foggy medium, must necessarily be subject to constant varia- 

 tion of visibility with every change of the atmosphere. The 

 change which I have to suggest, is to place a light of great 

 power about a mile in advance of the station, and at the same 

 time to limit its range by the depression of the resultant beam 

 within such a distance as to ensure its being visible at all 

 times. 



The arrangement I would propose for the attainment of 

 this object is remarkably simple, and consists in placing one 

 of Fresnel's annular lenses, illuminated by a gas or oil burner, 

 as may be most convenient, in a small chamber, glazed in 

 front, and supported on a stage of carpentry of sufficient size 

 to span the rails, and permit the train to pass under it ; but 

 the purpose might perhaps be equally well served by placing- 

 the stage at the side of the railway, and inclining the beam 

 obliquely to the line. In order to limit the range of the lens 

 to a short distance, and thereby to ensure the light being vi- 

 sible in all states of the weather at the same point, I would in- 

 cline the instrument, so that the length of the trajectory from 

 the lens to the observer's eye should not exceed about 700 

 feet, which falls far short of the distance at which the light of 

 the lens would be obscured even in the thickest fog. I may 

 remark that the inclination of the lens is too small to require 

 any correction in the position of the flame ; but this could be 

 easily accomplished if necessary, more especially when gas is 

 employed. In curved lines of railway the same effect might 

 in certain cases be produced by placing the lens on a level 

 with the observer's eye, and directing the refracted beam so 

 as to cut the railway obliquely. In this case the limitation 



VOL. XXX. NO. LX. ATIUL 1841. 7, 



