8^8 COSMOS. 



tions regarding the temperature otf the Earth. As all natural 

 phenomena — -all the changes to which terrestrial matter is 

 subject — are connected with modifications of heat, light, and 

 electricity, whether at rest or moving in currents, and as like- 

 wise the phenomena of temperature, acting by the force of 

 expansion, are most easily discernible by the sensuous percep- 

 tions, the invention and improvement of thermometers must 

 necessarily, as I have already elsewhere observed, indicate a 

 great epoch in the general progress of natural science. The 

 range of the applicability of the thermometer, and the rational 

 deductions to be arrived at from its indications, are as immeas- 

 urable as the sphere of those natural forces which exercise 

 their dominion over the atmosphere, the solid portions of the 

 earth, and the superimposed strata of the ocean — alike over 

 inorganic substances, and the chemical and vital processes of 

 organic matter. 



The action of radiating heat was likewise investigated, a 

 century before the important labors of Scheele, by the Floren- 

 tine members of the Accademia del Cimento, by remarkable 

 experiments with concave mirrors, against which non-lumin 

 ous heated bodies, and masses of ice weighing 500 lbs., act- 

 ually and ai^])arentlij radiated.* Mariotte, at the close of 

 the seventeenth century, entered into investigations regarding 

 the relations of radiating heat in its passage through glass 

 plates. It has seemed necessary to allude to these isolated 

 experiments, since in more recent times the doctrine of the 

 radiation of heat has thrown great light on the cooling of the 

 ground, the formation of dew, and many general climatic 

 modifications, and has led, moreover, through Melloni's admi- 

 rable sagacity, to the contrasting diathermism of rock salt 

 and alum 



To the investigations on the changes in the temperature of 

 the atmosphere, depending on the geographical latitude, the 

 seasons of the year, and the elevation of the spot, were soon 

 added other inquiries into the variation of pressure and the 

 quantity of vapor in the atmosphere, and the often-observed 

 periodic results, known as the law of rotation of the winds 

 Galileo's correct views respecting the pressure of the atmos 

 phere led Torricelli, a year after the death of his great teacher, 

 to the construction of the barometer. It would appear that 

 the fjict that the column of mercury in the Torricellian column 

 fitood high*)r at the base of a tower or hill than at its summit, 



* Autinoii, Saggi delV Accad. del Cim., 1841, D. M4, and in the Ag 

 /hinte at the end uf the bi)i)k, p. l.xxvi. 



