18G SCIENCE IX SHOUT CHAPTERS. 



not only of the permanent gases, nitrogen and oxygen 5 it con- 

 tains, besides these and carbonic acid, a considerable quantity 

 of gaseous matter, which is not permanent, but which may be 

 a gas at one moment contributing its whole weight to that of 

 the general atmosphere and at another moment some of it 

 may be condensed 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 barometer, 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, cloudy matter, or minute particles 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 or other metal vessel, fill 

 it with a mixture of salt with pounded ice or snow, and then 

 hold the outside of the canister 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 southwesterly wind, that lias 

 swept over the tropical regions of the Atlantic ocean, reaches 

 the comparatively cold shores 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 falls on the western coasts, on Cornwall, Ireland, 

 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 manner, 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 imme- 

 diately 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 condensation upon the barometer 

 is instantaneous ; the air begins to grow lighter immediately 



