468 



Motion affects Pressure in Barometer. 



making. Then, directly the rain has fallen and the air becomes quiescent, the 

 barometer rises again ; the influence of motion having ceased equilibrium is 

 nearly restored. In tropical countries, where rain is formed at a high elevation 

 above the ground, the horizontal movements in the upper strata are masked 

 completely by the actual rainfall near the surface, tropical rain making in its 

 fall vertical currents in the direction of the ground, thus causing the barometer 

 to rise because it actually puts an extra line of pressure on the cistern. In the 

 Doldrums, it is well known that the winds meeting near the sea-level con- 

 stantly mix and cause an upward current, accompanied by condensation of 

 vapour. The barometer is therefore always a little under 30 inches, not 

 because the atmosphere is lighter there than elsewhere, but because there is 

 horizontal motion across the column caused by the generation of rain, and 

 motion upwards caused by the expansion of the air vertically. The two 

 different causes produce the same effect in diminishing the actual pressure on 

 the cistern of the barometer. It is possible with a velocity of ten miles an hour, 

 in an artificial current of air or blast in a fan to depress the barometer cri inch, 

 independently of any rarefaction or condensation of the air itself. This would 

 cause a fall of 0*5 in. for a current of fifty miles an hour if the barometer fell 

 in like proportion. This rate would accord with that ob served in atmospheric 



ft 



storms when the force of the wind is fifty miles per hour. He thought no change 

 of mere weight of atmosphere could cause the barometer to fall 1 '693 in. as 

 happened in Guadaloupe in September, 1853; or at Bromley I'o in. on 

 November 29th, 1874 ; or i'2i in. at Guildford at the same time. This result 

 or fall, showing great diminution of pressure, was perhaps equally due to 

 sudden condensation of vapour causing local currents, and to the strong winds 

 derived from distant regions causing horizontal motion across the column of air, 

 and reducing pressure on the cistern of the barometer and causing the column 

 to shorten. Then, on the contrary, in London, on December 1st to 14th, 1873, 

 the barometer averaged 30-5 inches, the atmosphere being excessively still, and 

 fog continuous. Directly rain occurred, on the 15th, the barometer fell for 

 motion in the air. In his experiments on the Injector, described page 215, 

 Phil. Mag. 1874, he found that in the body of the Injector there was 121 lbs. 

 of pressure by the gauge, accompanied by rapid motion of the steam, yet, in 

 an adjoining tube, connected with the water tank opening at a right angle into 

 the Injector, there were two inches of vacuum according to a water gauge. 

 The experiment shows that within an inch of distance of a current of steam at 

 IOI lbs. pressure by the gauge, there was actually an open water pipe with a 



