THE BAROMETER AND THE WEATHER. 145 



densed into liquid particles that fall through it more or 

 less rapidly, and thus contribute nothing to its weight. 



What, then, is this variable constituent that sometimes 

 adds to the weight of the atmosphere and the consequent 

 height of the baromerer, and at others may suddenly cease 

 to afford its full contribution to atmospheric pressure? 



It is simply water, which, as we all know, exists as solid, 

 liquid, or gas, according to the temperature and pressure 

 to which it is exposed. We all know that steam when it 

 first issues from the spout of a tea-kettle is a transparent 

 gas, or true vapor, but that presently, by contact with the 

 cool air, it becomes white, cloiidy matter, or minute parti- 

 cles of water; and that, if these are still further cooled, 

 they will become hoar-frost or snow, or solid ice. Artificial 

 hoar-frost and snow may be formed by throwing a jet of 

 steam into very cold, frosty air. If you take a tin canister 

 qr other metal vessel, fill it with a mixture of salt with 

 pounded ice or snow, and then hold the outside of the can- 

 ister against a jet of steam, such as issues from the spout 

 of a tea-kettle, a snowy deposit of hoar-frost will coat the 

 outside of the tin. Now let us consider what takes place 

 when a warm south-westerly wind, that has swept over the 

 tropical regions of the Atlantic ocean, reaches the compar- 

 atively cold sliores of Britain. It is cooled thereby, and 

 some of its gaseous water is condensed forming mists, 

 clouds, rain, hoar-frost or snow. The greater part of this 

 forms and fulls on the western coasts, on Cornwall, Ire- 

 land, the Western Highlands of Scotland. Ireland gets 

 the lion's share of this humidity, and hence her " emerald " 

 verdure. The western slope of a mountain, in like man- 

 ner, receives more rain than the side facing the east. 



How does this condensation affect the barometer? 



It must evidently cause it to fall, inasmuch as the air 

 must be lightened to the exact extent of all that is taken 

 out of it and precipitated. But the precipitation is not 

 completed immediately the condensation occurs. It takes 

 some time for the minute cloudy particles to gather into 

 rain drops and fall to the earth, while the effect of the con- 

 densation upon the barometer is instantaneous; the air be- 

 gins to grow lighter immediately the gas is converted into 



