Semi-diurnal Fluctuations of the Barometer. 171 



there must therefore be successively more water evaporated 

 and thrown into the atmosphere to be added to its weight. 

 And according to the temperature theory, this water, now 

 converted into vapour, must, up to say 10 o'clock, press with 

 sufficient force on the mercury to counteract the lightening 

 influence of the rising temperature, as during that time the 

 barometer rises. 



From 10 until 1 o'clock, as the temperature rises still 

 higher, as compared with the wet-bulb thermometer and the 

 dew-point, evaporation must go on increasing, and the in- 

 crease of vapour pressure ought to continue; but it appears 

 from the table not to do so, as the mercury of the barometer 

 falls instead of continuing to rise; we have therefore to try to 

 ascertain what can be the cause of this fall, while additional 

 vapour is passing into the atmosphere. 



Those who advance the temperature theory, say that the 

 fall of the barometer is caused by the increasing temperature 

 of the atmosphere produced by the action of the sun on the 

 surface of the earth, and the air near to it; and they must main- 

 tain that this increase is sufficient, not only to lighten the atmo- 

 sphere enough to cause the fall of the barometer, but also in ad- 

 dition to counteract the influence of the increased vapour pres- 

 sure. Now at Plymouth the temperature rises from 5 to 10 a.m. 

 nearly 6°, and may be supposed to lighten the atmosphere to 

 a certain extent; at the same time evaporation throws vapour 

 into the atmosphere. We are, however, required to suppose 

 that the vapour produces so much greater effect by pressing 

 on the mercury, than the heating of the atmosphere does in 

 reducing atmospheric pressure, that the whole pressure be- 

 comes greater and the mercury rises. But after 10 o'clock 

 the temperature continues to rise, but in a smaller degree, say 

 nearly 3°, and vapour must be more abundantly thrown into 

 the air, as is shown by the extent to which the wet-bulb ther- 

 mometer is kept down ; yet the barometer, instead of conti- 

 nuing to rise, suddenly turns and falls, and continues falling 

 from 10 to 1 o'clock, the time of the highest temperature ! 

 So that according to this theory, from 5 to 10 o'clock, the 

 sun heats the air nearly 6° and produces some vapour; and 

 the two influences acting together cause the barometer to 

 rise, but from 10 to 1 the sun heats the air about 3°, and must 

 throw much additional vapour into the atmosphere; and then 

 these two influences still acting together cause the barometer 

 to fall ! This is attributing opposite effects to the same 

 causes, and must be presumed to be erroneous. 



But let us examine the valuable Plymouth tables a little 

 more minutely. The first column gives the temperature as 



