544 



LECTURE LVI. 



ON CLIMATES AND WINDS. 



THE science of meteorology relates principally to the natural history 

 of the air, and to such temporary changes in the earth and sea as are 

 produced by causes not mechanical only. The subject is of a very com- 

 plicated and intricate nature ; it comprehends many effects derived from 

 such causes as belong separately to every department of physics which we 

 have hitherto examined ; and although it lias occupied the attention of 

 several philosophers of considerable eminence, we cannot yet boast of 

 having made any great advancement in it. Whether we shall ever be able 

 to carry our theories to so high a degree of perfection, as to furnish us 

 with much information applicable to the purposes of common life, to agri- 

 culture, or to medicine, is at present uncertain ; although some advantage 

 has already been derived from the indications of meteorological instru- 

 ments ; and the philosophy of the science is in many respects much more 

 advanced than has commonly been supposed. We shall divide this exten- 

 sive subject into two parts, the first relating principally to the effects of 

 heat on the atmosphere, including the phenomena of winds ; the second 

 to the nature and consequences of evaporation, comprehending atmo- 

 spherical electricity, and to the effects of subterraneous fires and igneous 

 meteors. 



The variations of temperature in different parts of the earth's surface, 

 require to be examined in the first place ; since they are not only of con- 

 siderable importance in themselves, but are also among the principal causes 

 of other changes in the state of the winds and weather. These changes 

 are measured by thermometers, of various kinds, which have already been 

 described ; but, for meteorological purposes, some additions are frequently 

 made to the simple thermometer. In Six's thermometer,* the tube is 

 twice bent, so as to return in a parallel direction : the bulb is in the form 

 of a long cylinder, and is usually filled with spirit of wine, which is in 

 contact with a portion of mercury occupying the lower part of the tube ; 

 and this is succeeded by a second portion of spirit. The mercury carries 

 on each of its surfaces an index, which is retained in its remotest situation 

 by means of a weak spring ; and consequently shows the greatest degree 

 of heat or of cold that has happened since the last observation. The 

 indexes are of iron or steel, and may be brought back to the surface at 

 pleasure by means of a magnet ; they are carried up by the mercury, more 

 by its capillary action, than by the difference of the specific gravities. A 

 similar effect is obtained in Rutherford's t arrangement of a pair of ther- 

 mometers, one with mercury, the other with spirit of wine, placed in a 

 horizontal position ; one index being without the surface of the mercury, 



* Ph. Tr. Ixxii. Six on Meteorology, Maidst. 1794. 

 f Ed. Tr. iii. 247. 



