224 SCIENTIFIC APPARATUS. 



instrument is still much used in cold climates. It is exhibited 

 among the Russian instruments, and elsewhere. 



The Psychrometer, called also Mason's, or August's, or the 

 Dry and Wet Bulb Hygrometer, consists of two thermometers, one 

 of which has its bulb coated with muslin and moistened with 

 water. The theory of this instrument is explained in the " In- 

 structions in the Use of Meteorological Instruments," p. 47. 



PRECIPITATION. 



Rain is measured by means of a rain-gauge, which in its sim- 

 plest form consists of a funnel to catch the rain and a vessel to 

 receive it. This vessel may be either a bottle (Howard), or a 

 cylinder (Glaisher). It is then measured either by the use of a 

 graduated rule to measure the depth, or by pouring the amount 

 into a graduated glass cylinder. The various forms of rain-gauge 

 -are very numerous. 



Snow is usually collected in a rain-gauge, but such a measure- 

 ment is very deceptive, as owing to wind the flakes are often 

 blown out of the gauge, while in heavy falls the gauge is filled and 

 piled up with snow, so that the true amount cannot be well ascer- 

 tained. Babinet's gauge is provided with a lamp underneath to 

 keep the funnel hot and thaw the snow which falls. The mea- 

 surement of snow, by gauging the depth of the coating on a 

 level surface, is unsatisfactory, owing to the liability of snow to 

 drift. 



Rain-gauges are made self-recording in various ways, either by 

 a system of tilting buckets, which make a mark whenever each 

 bucket-full has been received and discharged, as in Crossley's 

 gauge. In Beckley's gauge, adopted by the Meteorological 

 Committee of the Royal Society, the bucket holds o. 2 inches ot 

 rain. It is carefully counterpoised, and as it descends, according 

 as the rain accumulates, it moves a pencil which marks a paper. 

 It therefore yields a continuous record. 



