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 south-westerly wind, that has 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 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 condensation upon the barometer is instantaneous; the air begins to grow lighter immediately the gas is converted into cloud or mist, and the barometer falls just at the same time and same rate as this is produced; but the rain comes some time afterwards. Hence the use of the barometer as a “weather glass.” When intelligently and properly used it is very valuable in this capacity; but, like most things, it may easily be misunderstood and misused.