Report of the Kew Observatory Committee. 387 



of the most distant of the selected objects which is visible at each 

 observation hour. 



Atmospheric Electricity. — The series of eye observations commenced 

 last year with a Portable Electrometer at certain points in the imme- 

 diate neighbourhood of the Observatory have been continued. The 

 results arrived at seem interesting in themselves, and are likely to 

 prove of service in interpreting and checking the records obtained 

 with the water-dropper and elecfcrograph. 



Aneroid Barometers. — The apparatus referred to in last Report was 

 delivered by the maker, Mr. J. Hicks, early in the year, and a large 

 number of experiments have been made and reduced; the results 

 have not yet been published. 



Nocturnal Radiation. — Regular observations of two minimum 

 thermometers freely exposed on grass, having shown that a constant 

 lowering of their zeros had been taking place for some years ; two 

 other minimum s have been obtained, and the four instruments are 

 now being daily observed under similar conditions. It is believed 

 that this lowering of zero is mainly caused by the exposure of the 

 bulbs to strong sunshine during summer. 



Thermometry. — A set of French hard glass thermometers, standard- 

 ised at the Bureau International, have been obtained, together with 

 a hypsometer of the Sevres pattern, constructed under the direction 

 of Dr. Guillaume. Some preliminary comparisons have been made 

 between the French thermometers and some Kew standards at tem- 

 peratures between the freezing point of mercury and 100° C. 



Platinum Thermometry. — This subject has of late years come into 

 prominence mainly through the memoirs* of Professor Callendar and 

 Mr. E. H. Griffiths. The conclusion they have reached is that sup- 

 posing " platinum temperature," pt, be denned so that its equal 

 increments answer to equal increments in the electrical resistance of 

 a pure platinum wire, the formula 



t-pt = a{(*/ioo) 2 -(*/ioo)}, 



where t is temperature on the air scale, and B a constant — for a par- 

 ticular sample of wire — holds, with at least a close approach to 

 accuracy, throughout a wide range of temperature. The convenience 

 of platinum thermometers for measurements of high temperatures 

 has been independently testified to by Messrs. Heycock and Neville 

 in their extensive researches in connexion with the freezing points 

 of metals and alloys. f Even in dealing with ordinary temperatures 

 the advantage possessed by a platinum thermometer, that it may be 

 read at a distance from the spot where the temperature is measured, 



* ' Phil. Trans. A.,' 1887, p. 161 ; 1891 pp. 43 and 119 ; 1893, p. 361, &e. 

 f ' Transactions Chemical Society,' 1895 p. 161. 



