26 Report S.A.A. Advancement of Science. 



When the air is unsaturated the barometric pressure may continually 

 increase, and the vapour tension may increase in the same ratio ; but 

 when the saturation point is reached the proportion no longer holds, 

 and the curve of vapour tension must necessarily be controlled by the 

 temperature of the air. At the dew point a fall of temperature 

 means in general a deposit of dew, or some condensation of moisture, 

 and therefore a decreased tension ; a rise of temperature in the 

 presence of water no less means evaporation. Consequently if we 

 are looking for relationships between barometric pressure and vapour 

 tension, over the ocean will not be the place to find it. 



Dalton's explanation of the diurnal oscillation of the barometer 

 alluded to before was partly based on suppositions respecting the 

 behaviour of the vapour of water which, under certain circumstances, 

 are undoubtedly realities. Since it seems not to be generally known, 

 and also refutes certain opinions which a later generation has credited 

 him with, I take this opportunity to quote it : — 



" Another fact respecting the variation of the barometer 

 indeed appears to be well established, by the attentive and 

 careful observation of that instrument. I mean the diurnal 

 variation first observed in the torrid zone, and since then traced 

 through the temperate zone, though it is there blended with other 

 and more powerful variations from other causes. 



" Generally speaking, the fact is this : that early in th-? 

 morning, about sunrise or soon after, the barometer is higher, all 

 other circumstances the same, than it is afterwards ; that it droops 

 a little as the heat of the day advances, and is lowest nearly in 

 the warmest part of the day ; after vvhich it rises as the air cools, 

 and in the evening nearly recovers what it had lost since morning. 



"The sun's power being greatest in the torrid zone, this effect 

 of it (for it is evidently an effect of temperature) is there a 

 maximum ; and on this account it is more conspicuous there, as 

 well as on account of the other variations being of less magnitude 

 than in the temperate and frigid zones. The effect diminishes, 

 in leaving the equator, in some proportion as the latitude, the 

 seasons, and other circumstances. 



" The sun is constantly heating the earth and air successively 

 from east to west : the air being heated expands in various 

 direction to restore an equilibrium of pressure ; if this expansion 

 was only in a perpendicular direction, it would not disturb the 

 barometer ; but as the air will go in any direction where the 

 pressure is least, it has a lateral motion as well as a perpendicular 

 one ; and hence the column pressing on the mercury is less in 

 quantity during the high temperature ; but when the excess of 

 temperature is withdrawn, the air falls back into its former 

 position. 



" It will be perceived that the principle we have adopted in. 

 the Essay on the Variation of the Barometer is that an equality 

 of elasticity in two vertical columns of air will in great part 

 counteract an inequality in their weight. To illustrate this. 



