DISCOVERIES IN THE CELESTIAL SPACES. 723 



now presented itself readily ; it may perhaps have been sug- 

 gested to Pascal, in a letter of Descartes *. It is not neces- 

 sary to enter into any especial explanation of the influence 

 exercised on the enlargement of physical geography and 

 meteorology by the barometer when used as a hypsometrical 

 instrument in determining the local relations of the earth's 

 surface ; and as a meteorological instrument in ascertaining 

 the influence of atmospheric currents. The theory of the 

 atmospheric currents already referred to, was established on a 

 solid foundation before the close of the seventeenth centuiy. 

 Bacon had the merit, in 1664, in his celebrated work entitled 

 Historia naturalis et experimentalis de ventis,] of considering 

 the direction of the winds in their dependence on thermome- 

 tric and hydrometric relations; but unmathematieally deny- 

 ing the correctness of the Copernican system, he conjectured 

 the possibility " that our atmosphere may daily turn round the 

 earth like the heavens, and thus occasion the tropical east 

 wind." 



Hooke's comprehensive genius here also diffused order and 

 light. J He recognised the influence of the rotation of the earth, 

 and the existence of the upper and lower currents of warm 

 and cold air which pass from the equator to the poles, and 

 return from the poles to the equator. Galileo, in his last 

 Dialogo, had indeed also regarded the trade winds as the 

 consequence of the rotation of the earth ; but he ascribed the 

 detention of the particles of air within the tropics (when 

 compared with the velocity of the earth's rotation) to a va- 

 pourless purity of the air in the tropical regions. Hooke's 



* Hen. Cartesii Epistolce, (Amstelod. 1682). P. iii. Ep. 67. 



t Bacon's Works by Shaw, 1733, vol. iii. p. 441. (See Cosmos, 

 pp. 321. 



J Hooke's Posthumous Works, p. 364. (Compare iry Rdat. Jiisto- 

 rique, T. 1, p. 199.) Hooke, however, like Galileo, unhappily assumed 

 a difference in the velocity of the rotation of the earth and of the at- 

 sphere : see Posth. Works, pp. 88 and 363. 



Although, according to Galileo's views, the detention of the particles 

 of air, is one of the causes of the trade winds, yet his hypothesis ought not 

 to be confounded, as has recently been done, with that of Hooke and Had- 

 ley. Galileo, in the Dialogo quarto (Opere, t. iv. p. 311), makes Salviati 

 say : " Dicevamo pur' ora che' 1'aria, come corpo tenue, e fluido, e 

 non saldamente congiunto alia terra, pareva che non avesse necessity 

 d'obbedire al suo moto, se non in quanto 1' asprezza della superficie ter- 

 restre ne rapisce, e seco porta una parte a se contigua, che di non molto 

 3 A 2 



