230 METEOEOLOGICAL OBSEEVATIONS. 



entered to the day on which they are observed. The rainfall is 

 read at 9 a.m., and entered for the previous day. 



Atmosphere. Enveloping the crust of the earth is a vast 

 invisible ocean of gaseous vapour, known to us as the air 

 or atmosphere. It is within this ocean that such wonderful 

 natural phenomena as rain, clouds, thunder, lightning, and 

 storms, so full of interest to the meteorologist, take place. By 

 the ancients air was considered to be an elementary substance, 

 but chemistry has demonstrated it to be a mixture of gases, the 

 two chief ingredients being nitrogen and oxygen. Traces of 

 other gases also occur, as will be seen by the following table ^ 

 which gives the average composition of normal air : 



Vols, per i,ooo 



. Nitrogen 779-0600 



Oxygen 



Aqueous vapour 

 Carbon dioxide 

 Ammonia 

 Ozone 

 Nitric Acid 



206-5940 

 14-0000 

 0-3360 

 0-0080 

 0-0015 

 0-0005 



1000-0000 



Like all gases the air is very elastic and compressible ^ 

 consequently its density, which is greatest at the earth's surface, 

 being pressed down by the mass above, rapidly diminishes as the 

 altitude increases, until at a height of about seven miles it 

 becomes so attenuated that breathing is impossible. 



It is important that the pressure or weight of the atmosphere, 

 which is never constant, varying in different places and at 

 different times, should be estimated, and for this purpose an 

 instrument known as the harometer (Greek haros, weight ; 

 metron, a measure), is used. 



Barometer. The standard barometer, which is hung on 

 the north wall of the Museum, has been compared with the 

 standard barometer at the Royal Observatory, Greenwich. It is 

 constructed on Fortin's principle, which has been proved to be 

 the most reliable, and consists essentially of three parts, (a) is a 

 glass tube (mounted in a brass frame) about 34 inches long, 



