696 



LECTURE LVL 



ON CLIMATES AND WINDS. 



TL HE 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 con)plicated and intricate na- 

 ture; it comprehends many effects derived from such causes, as belong sepa- 

 rately to every department of physics which we have hitherto examined; and 

 although it has 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 agriculture, or to medicine, is at present uncertain; al- 

 though some advantage has already been derived from the indications of me- 

 teorological instruments; and the philosophy of the science is in many re- 

 spects much more advanced than has commonly been supposed. We shall di- 

 vide this extensive 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, re- 

 quire to be examined in the first place; since they are not only of considera- 

 ble 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 



