703 LECTURE LVir. 



Mr. Dalton estimates the true state of the air with respect to moisture. Thus, if the 

 glass begins to be moistened when the water is at 40°, he infers from the known 

 elasticity of steam at that temperature, that the quantity of moisture contained in. 

 tlie air is equivalent to the pressure of a column of mercury about a quarter of 

 an inch in height; and if the actual temperature of the air be 50", the corre- 

 sponding elasticity of steam being a little more than one third of an inch, 

 the daily evaporation in such air will amount to about one ninth of an inch, 

 making 40 inches in the whole year. In fact, however, the air is usually 

 nioister than this, and the mean evaporation of all England is, according to 

 Air. Dalton, about 23 inches only. 



In hotter climates, and in particular situations, the evaporation may be 

 considerably greater. The Mediterranean Sea, being surrounded by land, is 

 more heated than the ocean, and the winds which blow over it are drier; con- 

 sequently its evaporation is greater than that of the Atlantic, and its specific 

 gravity is increased by the increased proportion of salt; so that at the straights 

 of Gibraltar, a current runs inwards at the surface and outwards near the bot- 

 tom, for the same reason as the air, when it is denser in a passage than in the 

 adjoining room, blows a candle towards the room at the lower part of the 

 door, and draws it towards the passage at the upper. Had there been a con- 

 tinual current inwards through the Straights, at all parts, the Mediterranean 

 must in the course of ages have become a rock of salt. It is indeed remarka- 

 ble that all lakes, into which rivers run without any further discharge, are 

 more or less salt, as well as lakes in general near the sea: but where a river 

 runs through a lake into the sea, it must necessarily, in the course of time, have 

 carried the salt of the lake with it, if it had ever existed. 



f» 



Experiments on the deposition of moisture, like those of Mr. Dalton, arc 

 liable to a slight inaccuracy, on account of the effects of an apparent elective 

 attraction, by means of which, some substances seem to attract humidity at a 

 temperature a little higher than others. Thus, a surface of metal often re- 

 mains dry, in the neighbourhood of a piece of glass which is covered with 

 moisture. It is certain that some substances attract moisture from the air, 

 even when the quantity which it contains is incomparably less than that 

 which would saturate it, since it is on this circumstance that the construc- 

 tion of hygrometers depends; and it is probably by a property somewhat si- 



