Description of the Kew Observatory. 147 



between the free/ing and boiling points of water. This apparatus is 

 still employed for ordinary meteorological instruments, a second 

 smaller one of similar type being now devoted almost exclusively to 

 clinical thermometers. 



For temperatures below the freezing point of water, down to 12 F. 

 or lower, use is made of freezing mixtures ; and a considerable 

 number of minimum thermometers are tested annually at the 

 ireezing point of mercury, which is reached by the aid of compressed 

 carbonic acid gas. 



For temperatures above the boiling point of water corrections are 

 .at present obtained by means of calibration; but experiments are in 

 progress which aim at replacing or extending the results so obtained 

 by direct observations in molten metal, or some other suitable 

 medium. 



In 1878 a hydraulic press was erected for testing deep-sea thermo- 

 meters mainly on behalf of the Admiralty under pressures similar 

 to those they experience at great depths. In the same year the 

 practice was introduced of etching a distinguishing mark a mono- 

 gram of KO on ordinary thermometers, whose errors do not exceed 

 certain limits. 



Barometers. The testing of mercury barometers dates from 1853, 

 and comprises the ordinary patterns used both on land, and sea. 

 There has been no such development of numbers here as has occurred 

 in the case of thermometers ; but, on the whole, only a slight in- 

 crease. The testing of aneroid barometers was instituted considerably 

 more recently, but in this case the number tested annually has shown 

 a considerable rise. 



Hydrometers. The testing of hydrometers also dates from 1853. 

 .At first the examination was practically limited to hydrometers 

 intended for ocean meteorology, but of late years there have been a 

 large number intended for commercial and excise purposes. 



Anemometers. These instruments are tried on a staging erected on 

 the observatory roof, at such a height as to place them on a level 

 with the standard Robinson anemometer. The records of each in- 

 dividual anemometer are compared with those of the standard for a 

 sufficient variety of wind velocities. 



Rain Gauges. Ordinary rain gauges have the mean diameters of 

 their receiving surfaces measured, and the necessary corrections 

 determined to the rainfall recorded. More complicated gauges of 

 the self-recording pattern are set up in the neighbourhood of the 

 standard Beckley gauge, and their records over a considerable time 

 compared with those of the standard. 



Sunshine Recorders. Only a small number of these instruments 

 have been examined. The method of testing comprises an examina- 

 tion into the proper working of the instrument, and a comparison 



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