ON THE PHYSICAL VIEW OF NATURE. 



99 



Chemical Science, though for merely external reasons 

 this was summarily handled. It is equally significant 

 that the first valuable suggestions as to the connection of 

 the various sciences, and the practical or common measure 

 of the various agencies, came from practical or professional 

 persons who took an outside and general view of physical 

 and chemical processes and their application in arts and 

 medicine. Young himself was a medical man, as were 

 Eobert Mayer and Helmholtz after him. Practical men 

 such as Watt felt the necessity of measuring not so much 

 forces (in the Newtonian sense) as the action of forces, 

 and introduced the term power, and the quantity called 

 horse-power 1 to measure the capacity of an engine for 

 doing work. Newton had already measured this action 2 



5. 



Watt intro- 

 duces the 

 term 

 "power." 



of the quantity of motion ; but 

 although this opinion has been very 

 universally rejected, yet the force 

 thus estimated well deserves a 

 distinct denomination." See also 

 p. 172. 



1 The quantity called horse- 

 power was introduced by Boulton 

 and Watt to measure the power of 

 the engines they built and sold at 

 Soho towards the end of the eigh- 

 teenth century. They caused ex- 

 periments to be made with the strong 

 horses used in the breweries in Lon- 

 don, and from the result of these 

 trials they assigned 33,000 lb., raised 

 one foot per minute, as the value of 

 one horse-power. Dr Young in his 

 'Lectures' has the following state- 

 ment: "A steam-engine of the 

 best construction, with a 30-inch 

 cylinder, has the force of forty 

 horses ; and since it acts without 

 intermission, will perform the work 

 of 120 horses or of 600 men, each 

 square inch of the piston being 

 nearly equivalent to a labourer" 

 (vol. i. p. 103). 



2 See the Scholium to the " Axio- 



mata sive Leges Motus," p. 25 of 

 the first edition of the ' Principia,' 

 in which the ' ' Agentis Actio " is 

 measured " ex ejus vi et velocitate 

 conjunctim." Thomson and Tait 

 (' Natural Philosophy,' 1886, part i. 

 p. 250 sqq., and Tait, 'Dynamics,' 

 1895, p. 181) have drawn attention 

 to the fact that this passage of the 

 ' Principia ' contains implicitly the 

 modern notion of energy, and the 

 principle of the conservation of 

 energy. The continental historians 

 named above are inclined to give 

 Huygens credit for having first 

 made explicit use of the idea of the 

 conservation of the quantity now 

 termed energy, and they trace the 

 further elucidation of it to the 

 Bernoullis, especially John Ber- 

 noulli, who repeatedly speaks of the 

 " conservatio virium vivarum," and 

 "urges that where vis viva dis- 

 appears, the power to do work 

 (fac'ultas agendi) is not lost, but is 

 only changed into some other 

 form" ('Opera,' 1742, vol. iii. pp. 

 239 and 243, quoted by Planck, loc. 

 cit., p. 10). 



