142 Record of the Royal Society. 



Soc. Proc.,' vol. 6, pp. 178 188) ; the dividing engine is used inter 

 alia for calibrating and dividing mercury thermometers. 



Computing Room. This is a large octagonal room, similar to the 

 North Hall, in which the greater part of the reduction of meteoro- 

 logical observations is effected ; it contains a standard clock by 

 Shelton (11). 



West Boom. This contains the Thermograph (12) and Electro- 

 graph (13). 



The Thermograph was designed and fitted up by Beckley under 

 the direction of Dr. Balfour Stewart, in 1866 (see 'Report of the 

 Meteorological Committee,' 1867). The recording part consists 

 essentially of a drum carrying photographic paper which is driven 

 round by clockwork once in forty-eight hours. There are twa 

 thermometers, a dry bulb and a wet bulb, of as nearly as possible 

 the same pattern. Their bulbs are outside the building but inside a 

 screen. The stems first pass horizontally through the wall, and 

 then bend at right angles so as to pass vertically in front of two 

 fixed mirrors. The mirrors reflect beams of light which emanate 

 from gas jets, and are transmitted through condensing lenses. The 

 mercury column of each thermometer has a small air bubble en- 

 trapped in it, and only where the bubble is can light pass through 

 to the photographic paper. The bubble rises and falls with tho 

 mercury, and thus the photographic trace gives a continuous record 

 of its height, and so indirectly measures the temperature. Along- 

 side the thermograph thermometers in the screen, there are ordinary 

 wet and dry bulb standard thermometers, which are read daily at 

 10 A.M., noon, 2 P.M., 4 P.M., and 10 P.M. These readings serve to- 

 standardise the thermograph curves. There are also in the screen 

 ordinary maximum and minimum thermometers for use in case of 

 failure of the thermograph. 



The Electrograph (13), invented by Lord Kelvin, consists of a 

 quadrant electrometer containing a needle to which a small mirror is 

 attached ; from this a beam of light is reflected on to photographic 

 paper. The needle is connected electrically with the Water- 

 Dropper (14). The curve obtained is intended to show variations in 

 the electric potential at the point outside the building where the jet 

 from the water-dropper breaks into drops. The curve is standardised 

 from time to time by means of a portable electrometer. The position 

 of the base line answering to zero potential is determined by direct 

 experiment daily at 11 A.M. and 4.30 P.M. 



Near the electrograph is the chronometer oven (15), employed for 

 testing Navy chronometers at a moderately high temperature ; the 

 temperature is regulated by means of a Kullberg's governor. 



Thermometer Room. The Thermometer Comparing Apparatus 

 (16) was designed by Mr. F. Galton. The thermometers to be tested 



