February 8, 1895.] 



SCIENCE. 



153 



Aliout 1850, however, the accumuhiting 

 data of experimental philosophers and the 

 relleetions of a number of theorists led to 

 the annoimcemeut of the principle of the 

 conservation of energy, a doctine which is 

 now hekl to be tlie highest generalization of 

 mechanical science. This doctrine asserts 

 that tlie total energy of any mechanical sys- 

 tem is a quantity which can neither be in- 

 creased nor dimislied by any mutual action 

 of the parts of the system, though it may 

 be converted into any one of the forms of 

 which energy is susceptible. Thus, the 

 sohir system, supposing it to be isolated 

 fi-om all other systems of the universe, 

 contains a detinite amount of energy, and 

 whatever may have been or may be the 

 vicissitudes of the sun and planets, that 

 quantity of energy was and will be the 

 .same. 



But what, in common parlance, some one 

 may properly enquire, is energj^ in a me- 

 chanical sense ? Tlie answer to this question 

 is not difficult. If we raise a weight, as, 

 for example, an elevator car above the sur- 

 lace of the earth, work must be done. On 

 the other hand, if it be elevated and its cable 

 be cut, the car will fall back to the earth 

 aiul do work of destruction in its fall. The 

 work stored up in raising the car to a given 

 height is called energy of position, or poten- 

 tial energy. The work the car can do by 

 reason of its fall is called energy of motion, 

 or kinetic energy. If a strict account of 

 the expenditure is kept in this case, it is 

 found that the sum of the energies of posi- 

 tion and motion at any instant is constant. 

 Similarly, it was found bj' Count Rumford 

 and .Joule that in boring cannon and in agi- 

 tating li([uids heat is produced, and that if 

 in these cases accurate record is kept, the 

 amount of lieat developed bears a definite 

 ratio to the amo\nit of energj' expended. 

 Thus heat is brought into the category of 

 energy, hot bodies being such, as we now 

 tliiuk, by reason of the more or less furious 



agitation, or kinetic enei^y, of tlieir ulti- 

 mate particles. 



The law of the conservation of energy, 

 then, is a simple statement of Nature's 

 balance-sheet with respect to material sys- 

 tems. Tlie capital invested remains always 

 the same, however divereified may be the 

 investments. A part may be entered as 

 potential energy; a part as kinetic energy; 

 a part as heat: etc., but when properly ad- 

 ded together, their sum is constant. Broadly 

 speaking, it is believed that the various 

 forms of energj' may be comprised in two 

 categories: the energy of position, or poten- 

 tial energy, and the energy of motion, or 

 kinetic energy. 



It is interesting to note in connection 

 with the history of this doctrine that tlie 

 ideas w-hich led up to it go back certainly to 

 the time of Newton and Leibnitz. The 

 conservation of matter is, indeed, a funda- 

 mental concept of mechanics; but the earlier 

 philosophers, from Newton and Leibnitz 

 down, were acquainted with the conserva- 

 tion of momentum and energy in a variety 

 of special cases. And it is probable that 

 our modern science owes something to the 

 nietaphy.sical notions of Descartes. Mauper- 

 tius and others, who held that Nature per- 

 forms her operations in the most economical 

 ways and is, on the whole, conservative. 



It appears not a little remarkable that 

 this important doctrine eluded the insiglit 

 of Lagrange and Laplace. Lagrange, espec- 

 ially, was so near to it that he supplied 

 iiearlj' all the analytical machinery essential 

 to put it into practical use. Indeed, tliat 

 machinery meets a much higher demand. 

 It not only enables us to express and in- 

 terpret the properties of systems which are 

 obviously mechanical, but it shows clearly 

 what must be the characteristic features of 

 a mechanical exi)lanation of any phenom- 

 enon. Thus, in the direct application of the 

 doctrine of energy to a mechanical system, 

 we express the kinetic energy in terms of 



