14 THE AIR 



dexterity are required in setting the vernier so as to 

 allow for this pumping, and even in the best instru- 

 ments, with the most alert and patient observer, it 

 is impossible to read with anything like the degree 

 of accuracy which may be obtained at stations on 

 land. The rise and fall of the ship with the waves 

 produces a smaller irregularity, and when it is re- 

 membered that, roughly speaking, the barometer falls 

 one-thousandth of an inch for every foot of increase of 

 height, it is plain that the vertical rise and fall of a ship 

 in rough weather may affect the readings by several 

 hundredths of an inch. It is necessary, therefore, in 

 any attempt at exact work, to take the mean of the 

 highest and lowest readings when the ship is in the 

 trough and on the crest of successive waves. It must 

 be remembered that a greater degree of accuracy in 

 reading the barometer is required for meteorological 

 purposes than when the barometer is merely used as a 

 nautical instrument to give warning of storms. The 

 barometer must be kept as free as possible from arti- 

 ficial heat and from risk of jars or knocks ; but when it 

 is suspended in suitable gimbals it does not require 

 any extra springs or strapping. 



2. The aneroid is a useful supplementary instru- 

 ment ; as it is more sensitive than the mercurial 

 barometer, it gives earlier warning of approaching 

 weather. It is free from the surging error due to the 

 rolling and pitching of the ship ; but no barometer 

 capable of measuring differences of pressure can 

 escape from the disturbances due to the pumping pro- 

 duced by suction of wind and to the rise and fall of 

 the waves. A barograph, or self-registering form of 

 aneroid, is, perhaps, the most suitable on board ship. 

 If hung with a spring hook to the roof of a cabin, 

 gives a continuous record, of the utmost value wh< 



