1877.] 



Thermometers at the Kew Observatory . 



85 



with hot water and standing on a turntable, in which a brass frame was 

 placed. 



The thermometers were attached to this framework, and the observer 

 having well agitated the water with a plunger, read the instruments in 

 succession through the glass as he turned the jar round before him, 

 reading each thermometer as it passed. He first turned it round from 

 right to left, and then back again from left to right. Each thermometer 

 was thus read twice, and the mean of the pair of readings was taken. 

 It is obvious that if the rate of cooling of the water be uniform, and if 

 the thermometers are observed at precisely equal intervals, the mean of 

 every pair of observations would be strictly referable to the temperature 

 of the water at the same moment of time, namely, to that which is half- 

 way between the beginning and end of the entire set. It is needless to 

 point out that these conditions can never be strictly fulfilled, although, 

 notwithstanding the imperfection of the process and the coarseness of the 

 apparatus, the observers acquired much certainty and skill in its mani- 

 pulation. Still the time occupied was unnecessarily great, and the chance 

 of error, owing to variations in the rate of cooling of the water, was 

 larger than it need be. Partly owing to this latter reason, and partly to 

 the fact that the number of thermometers sent to be tested has con- 

 siderably increased (being now not less than 3000 annually), I thought 

 it advisable to design and propose to my colleagues of the Kew Committee 

 the construction of an instrument of a much more substantial and ade- 

 quate character ; and to this the Committee assented. I was subsequently 

 indebted for many suggestions to Mr. De La Rue, and also to Mr. 

 Munro, of 24 Clerkenwell G-reen, London, by whom it was finally made.. 

 It has now been at work for two years, and its performance is quite 

 satisfactory ; experience has in the mean time suggested a few emendations 

 and simplifications, and I will therefore describe the instrument as at 

 present in use. 



The apparatus (see figs. 1 & 2) consists essentially of four parts 



(1) A water-vessel. 



(2) An agitator, worked by a handle on the eutside.. 



(3) An external heating arrangement. 



(4) A frame on which to hang the thermometers, turned by a 

 handle on the outside. 



(1) The Water-vessel 



This is a cylinder of stout copper, 2 ft. 2 in. high and 1 ft. in diameter. 

 In its base there is a central aperture through which the concentric 

 vertical axes are passed, which respectively carry the agitator and the 

 thermometer frame ; the top of the cylinder is entirely open ; a vertical 

 slit, 1 ft. 10 in. long and 4^ in. wide, is cut in the side of the cylinder 

 and the slit is glazed with a stout sheet of plate-glass, the joints being 

 made water- and steam-tight by means of india-rubber packing. 



