Notes. 437 



By observing regularly the height of the barometer for a con- 

 siderable period in the same place, we find that it does not remain 

 constantly the same. For some time after the instrument was in- 

 vented it was supposed that the mercury stood higher just before 

 rain, and lower during fair weather. Reasons were assigned for this 

 supposed fact It was said, that when it is about to rain the air is 

 charged with water, and consequently that the weight of the atmo- 

 sphere is more considerable ; and that, on the contrary, this weight 

 must be less in fair weather, because the air is then relieved from 

 a certain part of the moisture contained in it Unfortunately for 

 this hypothesis it has been found, more recently, that the quantity of 

 water which the air is capable of containing, increases with the 

 temperature, so that in summer it contains, for the most part, more 

 water than in winter, although there is less fair weather in winter 

 than in summer. It appears also that the vapour of water is lighter 

 than the same volume of air, when the same elastic force is exert- 

 ed ; that is, if we substitute for a cubic foot of air, taken at a certain 

 height in the atmosphere, a cubic foot of aqueous vapour, of the 

 same temperature and elasticity, the vapour will weigh less than 

 the air, and will consequently exert less pressure upon the barome- 

 ter. We hence draw a conclusion the reverse of that which the 

 first observers of the barometer undertook to maintain, namely, that 

 the rise of the barometer indicates fair weather, and its fall, rain. 

 This is in fact agreeable to observation in ordinary cases. But it must 

 be confessed that the reason now given is but little better than that 

 we have been combating. 



The variations of the barometer are different in different places. 

 They are almost nothing upon the tops of high mountains, and be- 

 tween the tropics ; even in the temperate zones they are never 

 very great in calm weather. But the barometer almost always de- 

 scends rapidly before a violent storm, great changes taking place in 

 a few hours. On this account the instrument is particularly useful 

 at sea. 



By comparing observations made at different and remote places, 

 we discover a remarkable correspondence, which shows a simuUa- 

 neousnessin the motions of the atmospheric strata that would hardly 

 have been expected. Still this correspondence is far from being 

 perfect, especially as to the quantity of the change. 



By examining a long series of observations made in the same 

 place, we shall perceive amid all the accidental irregularities, that 

 there is a general tendency, occuring periodically, to rise and fall 

 at certain hours of the day. By a long series of observations, direct- 

 ed to this point, M. Raymond discovered, that in France the barome- 



