BAROMETERS 13 



deal to be supplemented when opportunities are offered 

 to scientific observers. 



The most important of all meteorological instru- 

 ments at sea is the barometer, for all our knowledge 

 of the cause and general prevalence of wind and the 

 broad lines of atmospheric circulation depends upon 

 exact and multitudinous measurements of pressure. 



i. The mercurial barometer must be hung in a 

 vertical position, and should be placed in a room with 

 a temperature as equable as possible, free from 

 draughts, in a good light, and at a convenient height, 

 so that the eye can be brought easily to the level of 

 the top of the mercury column. These conditions are 

 just as imperative at sea as on land, though much more 

 difficult to fulfil. At sea a vertical position can only 

 be preserved by a barometer so freely suspended that 

 the ship can roll without affecting it. Even with the 

 best arrangement of gimbals this is practically im- 

 possible, except in a very smooth sea, and the mercury 

 in the ordinary barometer surges up and down so as 

 to make accurate measurement extremely difficult. 

 The surging of the mercury due to the ship's motion 

 is checked in marine barometers by a constriction of 

 the tube at the cistern end, so that the action is 

 slowed down. A more serious cause of disturbance 

 arises from the action of the wind during heavy 

 weather. The barometer is most conveniently hung 

 in the chart-room of a modern steamer, from 20 to 

 60 feet above the sea-level. In this room the wind- 

 ward door is kept shut and the leeward door open ; 

 and as each gust of wind sweeps past, the air is sucked 

 out through the open door, and the mercury in the 

 barometer sinks to the diminished pressure, rising again 

 in the lull, and continuing to swing up and down 

 while the " pumping " action lasts. Great skill and 



