EQUIVALENT CAUSES. 417 



effect of two opposite tendencies to motion. Multitudes 

 of cases of this kind continually occur. 



Preventives are contrary or incompatible causes, and 

 if we know what circumstance prevents, or is incom- 

 patible with an effect, we are very near to knowing the 

 cause or condition of that effect, because we know it is an 

 exactly contradictory one. 



Forces, or dynamic causes, are convertible into each 

 other ; but probably not wholly so in every case. We 

 can convert mechanical force wholly into heat, but not the 

 reverse ; and hence arises the universal diffusion of force in 

 the form of uniform temperature, and the theory of dissi- 

 tpation of energy. 1 



Different forms of energy are not only convertible, but 

 I may be equivalent to each other. Equivalent causes are 

 I those which are equal in the amount of their energy ; thus, 

 | the amount of heat which must be imparted to one pound 

 | of liquid water in order to raise its temperature one Fahren- 

 heit degree would, if wholly converted into mechanical 

 power, be sufficient to lift a pound weight 772^ feet high. 

 The chemical power also stored up in a piece of coal 

 would, when converted wholly into mechanical force, be 

 sufficient to lift that piece of coal a height of more than 

 2,000 miles ; and the combustion of half a pound of car- 

 bon would thus be equivalent to lifting a man to the top 

 of the highest mountain. 



It is a partially false doctrine, that a cause is neces- 

 sarily similar to its effect ; but it is very flattering to hu- 

 man vanity to believe that that which made a man must 

 be like a man. There are, on the one hand, numerous 

 cases in which causes and their effects are similar, but, on 

 the other, there are multitudes of instances in which they 

 are largely different. The maker of a house is not fashioned 



1 See pp. 162-163. 

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