W WEATHER AKD WEATHER-GLASSES. 



the air in the atmosphere lie within foirr miles of the earth's surface. Its 

 height, supposing it to be of uniform density, would be about five miles : 

 but it is considerably higher, probably about 50 miles. First among the 

 causes affecting the atmosphere, and giving rise to winds, rain, and other 

 phenomena, is heat. Heat causes air to expand. Hence in the tropics there 

 is a powerful upward current of hot air, and a consequent rush of cold air 

 from the poles to supply its place. If the earth were at rest, and its 

 surface occupied by water, we should have but two great winds or 

 currents of air — one from the equator to the poles, and the cold current 

 from the poles to the eqiiator. Mountain ranges, elevated plains, deserts, 

 vast masses of land, extensive forests — all combine to break the uniformity 

 of the temperature of the globe, independently of the direct action of the 

 sun. For instance, a current of warm air loaded with vapoiu", coming in 

 contact with the lofty ridge of Blackstone Edge, would leave much of its 

 weight on the Lancashire side, and reach us as a much di-ier wind. The 

 precise nature of the influences of the great forces electricity, magnetism, 

 &c , is not so easUy described. Indeed, much must yet be done before we 

 shall be in a position to say positively which is the caiise and which the 

 effect imder given conditions. 



2. Construction of the Weather-Glass or Barometer. 



A weather-glass is a rude barometer. The barometer is made with 

 the most scrupulous care, and, after certain corrections, varying in each 

 case, becomes an exceedingly trustwoithy and beautiful instrument. It is 

 intended to measure the ever-varying weight of the air, and is thus con- 

 structed. A glass tube about 32 inches long, and closed at one end, is 

 fiUed with pure mercury ; the open end is tlien stopped with the finger, 

 and dipped beneath the surface of mercury contained in a glass or wooden 

 vessel. The instant the finger is removed, the fluid within the tube falls 

 an inch or two, and then remains at rest. Thus, as there is no air within 

 the tube to press the mercury loAver, it is supported by the pressure of the 

 air without upon tlie surface of the fluid in the cistern, ffhe distance from 

 the level of the mercury in the cistern to the top of the column within the 

 tube is then carefully measured, and marked upon the brass or wooden 

 support. To render the instnmient valuable as a scientific indicator of the 

 pressure of the air, many corrections are required, depending upon the 

 bore of the tube, the temperature of the room, the mode of moimting, &c , 



Influence oj elevation, &c. — If the barometer be placed at the sea- 



