98 THE AIM AND ACHIEVEMENTS OF SCIENTIFIC METHOD. 



felt of another concept to replace the weight of a body as 

 a measure of what has vaguely been thought of as the 

 " quantity of matter " in it. Examined critically, this need 

 is found to resolve itself into an instinctive demand for some 

 constant coefficient that shall characterize the behaviour of 

 the same body in all " dynamical transactions "* into which it 

 enters, including those in which it presents the property of 

 a variable weight. Investigations similar to those which 

 Huygens and his English contemporaries carried out upon 

 colliding bodiesf supply the^best materials for a clear answer 

 to the demand thus expressed, and lead at the same time to 

 a perfectly general and satisfactory concept of force. If two 

 inelastic bodies moving directly towards one another with 

 equal velocities of any given value bring one another to rest, 

 it is found that however their velocities are changed in amount, 



* Clerk Maxwell's happy phrase. 



t No. 43 of the Philosophical Transactions (January 11, 1668-9) con- 

 tains an enunciation of Dr. Wallis's General Laws of Motion (pp. 864-6), 

 and of Wren's Lex Naturae de Collisions corporum (pp. 867-8). Wallis 

 dealt with inelastic bodies ; Wren (like Huygens) with ideally elastic 

 bodies. In No. 46 (April 12, 1669) Regulce de Motu Corporum ex mutuo 

 impulsu, "communicated by Mr. Christian Huygens in a letter to the 

 Royal Society " were published, like the communications of Wallis and 

 Wren, " in the language of the learned " for the benefit of Continental as 

 well as of .English inquirers. In connection with the hypothetical course 

 of investigation followed in the text it is interesting to note that Wren 

 had' formed a concept of " velocitates corporum proprise et maxime 

 naturales [quse] sunt ad corpora reciproce proportionates." The property 

 of such velocities is that "[duo] corpora, R et S, habentia proprias 

 velocitates, etiam post impulsum retinent proprias ;" while a general law 

 (for elastic bodies) is expressed in the form, " Quantum R superat et S 

 deficit a propria velocitate ante impulsum, tantum ex impulsu abstrahitur 

 ab R et additur ipsi S, et e contra." 



Huygens, on the other hand, reaches, in Regula 8 (p. 928) the 

 general law of the text in the form " Quantitas motus [i.e., momentum] 

 duoruni corporum augeri minuive potest per eorum occursum ; et semper 

 ibi remanet eadem quantitas versus eandum partem, ablata inde quanti- 

 tate motus contrarii." 



It is instructive to notice, also, that Wren uses for the modern 

 " mass " the unexplained term " corpus," while Huygens explicitly states 

 of his bodies that " eorum moles oestimatur ex pondere." 



