22 



Photography at Greenwich Observatory. 



and light lever, the greater part of the weight of which and of 

 the float is counterpoised, leaving a small residue only pressing 

 on the mercury. The barometer is about thirty inches from 

 the cylinder, and the fulcrum of the lever is still further off. 

 The lever carries a plate of opaque mica in front of the 

 rotating vertical cylinder, having a small hole in it, through 

 which the photographic light of a gas jet, concentrated by a 

 cylindrical lens, passes, and records the rise and fall of the 

 barometer, which, by the action of the lever, is multiplied four 

 times in length, rendering the indications easily read off. An 

 independent pencil of light, shining through a fixed aperture, 

 traces a base line, from which the heights are measured, a few 



V 



^ 



H 



Fig. 2. 



eye observations being made every day to obtain fundamental 

 points. 



The annexed diagram (Fig. 2) shows this apparatus in 

 detail. AB is the barometer, the cylinder, T> the time- 

 piece, E the vertical rod, F the fulcrum, G the counterpoise, 

 H the lever and mica plate. 



Another vertical cylinder, placed under a shed in the 

 grounds of the observatory, photographically records the read- 

 ings of dry and wet bulb thermometers by a very simple 

 arrangement. 



