METEOROLOGY 



43 



Snow consists of a fall of " feathery ice crystals," while a fall 

 of " rain and snow together or snow partly melted " is termed sleet. 



PRESSURE. Instruments for determining the pressure of the 

 atmosphere are known as barometers. 



That the atmosphere exerts pressure can be shown by taking a 

 glass tube of a length of about 32 inches, closed at one end and com- 

 pletely filled with mercury ; when the open end is opened vertically 

 over a reservoir of mercury, it will be found that the mercury in the 

 tube settles at a level of about 30 inches above the 

 free surface of the liquid in the reservoir. Thus 

 the 30 inches of mercury are supported by the pres- 

 sure of the atmosphere and afford a measure of the 

 atmospheric pressure at that instant (Torricelli's 

 experiment). 



Forms of mercury barometers are adaptations 

 of the above. 



The Fortin Barometer consists of a glass tube 

 of about 80 cm. length, closed at the top end and dip- 

 ping into a vessel of mercury. There is a fixed 

 measuring scale, and the mercury in the vessel can 

 be adjusted to the zero of this scale by a screw whicli 

 alters the level of the mercury in the vessel till it 

 coincides with the zero. This is done by raising or 

 lowering the bottom of the vessel, which consists 

 of a piece of leather. This adjustment must be 

 made before a reading is taken, and, when it has 

 been carried out, the level of the upper surface of 

 the mercury can be read by means of the scale and FlG - 5. Fortin's 

 vernier attached. 



The Kew Pattern Standard Barometer has the diameter of the 

 tube contracted along a great part of its length to obviate the dis- 

 turbing effects of oscillation or " pumping." It is thus admirably 

 suitable as a marine barometer. Moreover to save periodical 

 adjustment of the level of the mercury in the reservoir it has a scale 

 of " shortened " inches, which takes account of the relative 

 capacities of tube and reservoir. 



The Aneroid Barometer does not contain liquid but consists of 

 a metal vessel, usually corrugated in form, from which the air has 

 been partially removed. The pressure of the atmosphere causes 

 the vessel to tend to collapse, but a spring, attached to it, obviates 

 this. By means of a suitable motion, consisting of a system of 

 levers, the changes in .the barometric pressure can be recorded on 

 a scale by a pointer. 



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