NATURAL PHILOSOPHY. 195 



to that of water, and the heat be supposed to bo drawn uniformly from 

 its entire mass, its temperature would thereby iindcrgo a diminution of 2 

 4' Fahr. annually. On the other hand, there is a vast store of force in our 

 system capable of conversion into heat. If, as is indicated by the small den- 

 sity of the sun, and by other circumstances, that body has not yet reached 

 the condition of incompressibility, we have, in the future approximation of 

 its parts, a fund of heat probably quite large enough to supply the wants of 

 the human family to the end of its sojourn here. It has been calculated that 

 an amount of condensation, which would diminish the diameter of the sun 

 by only the ten-thousandth part, would suffice to restore the heat emitted in 

 2000 years. Again, on our own earth, vis viva is destroyed by friction in the 

 ebb and flow of every tide, and must therefore re appear as heat. The amount 

 of this must be considerable, and should not be overlooked in any estima- 

 tion of the physical changes of our globe. According to the computation 

 of Bessel, 25,000 cubic miles of water flow in every six hours from one 

 quarter of the earth to another. The store of mechanical force is thus di- 

 minished, and the temperature of our globe augmented by every tide. "We 

 do not possess the data which would enable us to calculate the magnitude of 

 these eifects. All that we know with certainty is, that the resultant effect of 

 all the thermal agencies to which the earth is exposed has undergone no per- 

 ceptible change with the historic period. We owe this fine deduction to Ar- 

 ago. In order that the date palm should ripen its fruit, the mean tempera- 

 ture of the place must exceed 70 Falir. ; and, on the other hand, the vine 

 cannot by cultivated successfully when the temperature is 72 or upwards. 

 Hence, the mean temperature of any place at which these two plants nour- 

 ished and bore fruit must lie between these narrow limits, i. e. could not dif- 

 fer from 71 Fahr. by more than a single degree. Now, from the Bible we 

 learn that both plants were simultaneously cultivated in the central valleys of 

 Palestine in the time of Moses ; and its then temperature is thus definitely 

 determined. It is the same at the present time ; so that the mean tempera- 

 ture of this portion of the globe has not sensibly altered in the course of 

 thirty-three centuries. 



The future of physical science seems to lie in the path upon which three of 

 the ablest British physicists have so boldly entered, and in which they have 

 already made such large advances. I may, therefore, be permitted briefly to 

 touch upon the successive steps in this lofty generalization, and to indicate 

 the goal to which they tend. It has been long known, that many of the 

 forces of nature are related. Thus, heat is produced by mechanical action, 

 when that is applied in bringing the atoms of bodies nearer by compression, 

 or Avhcn it is expended in friction. Heat is developed by electricity, when 

 the free passage of the latter is impeded. It is produced whenever liyht is 

 absorbed ; and it is generated by chemical action. A. like intcrchangeability 

 probably exists among all the other forces of nature, although in many 

 the relations have not been so long perceived. Thus, the development of 

 electricity from chemical action dates from the observations of Galvani ; and 

 the production of magnetism by electricity from the discovery of Oersted. 

 The next great step was to perceive that the relation of the physical forces 



