1877.] 



Thermometers at the Kew Observatory. 



87 



The cylinder is placed inside a wooden box, taller than itself, and 1 ft. 

 5 in. square at its base, the space between it and the sides of the box 

 being filled with sawdust, whilst the exterior of the box is completely 

 covered with kamptulicon, in order to retain the heat of the water in 

 the enclosed vessel as much as possible. An aperture somewhat larger 

 than that in the cylinder is cut in the side of the box in front of it, and 

 is also glazed with plate-glass. 



A lid, containing 3 inches of sawdust, covered with a sheet of kamp- 

 tulicon, can be shut tightly down on the top of the cylinder and box, the 

 escape of the vapour given off during heating being provided for by 

 means of a steam-pipe. 



Pipes lead from the top and bottom of the water-vessel to an exterior 

 pipe ending in a funnel above and a cock below, so that water may be 

 poured in or drawn off from the vessel as desired. 



The whole is firmly fixed to a stout wooden stand about 2 ft. high. 



(2) The Agitator (see fig. 2). 



A stout and hollow brass axis, 2| in. in diameter, passes vertically up 

 through the centre of the base of the water-vessel, carrying three sets of 

 helical vanes, one above the other, arranged so that the upper and lower 

 vanes form segments of right-handed screws, whilst the intermediate 

 vanes are left-handed. The inclination of every vane is adjustable. 



The lower end of the axis passes through a stuffing-box in the bottom 

 of the cylinder, and is connected by gearing to a crank-handle projecting 

 outside the apparatus. It can be turned easily by the hand of the 

 observer, who thereby is able to agitate the water throughout the whole 

 depth of the vessel. 



(3) The Heating-Apparatus (see fig. 1). 



This is a copper tube 0*6 in. in diameter, which, issuing from the 

 back of the water-vessel near the bottom, is carried through the wooden 

 casing of the instrument, and is then coiled into a vertical spiral of six 

 turns, gradually diminishing in diameter. The end of the tube is after- 

 wards brought back into the water-vessel. 



A cluster of Bunsen burners being placed beneath the coil serves to 

 heat it and to make the water circulate inside the cylinder, thus warming 

 the whole of its contents. 



Experiment shows that, with the small coil used, 10 gallons of cold 

 water can be boiled in about six hours from the time of lighting the gas ; 

 in practice, however, when it is required to test thermometers near the 

 boiling-point only (mountain thermometers, for example) the apparatus 

 is filled with boiling water out of kettles put on an ordinary fire. 



A cone of sheet-copper is usually placed round the coil as a jacket, in 

 order to retain the heat from the gas-burners as much as possible ; this 



