v.J PROFESSORIAL LIFE. 97 



lectures, as difficulties arose, or interesting questions 

 suggested themselves, Forbes gave his thoughts to 

 Dr. Whewell, as in the following letter : 



' July 20th, 1833. 



' . . . Since I came home I have been at work upon 

 fundamental principles of Dynamics. I confess I 

 am half afraid to write to you upon your hobby, but T 

 do want to put one or two questions to you about 

 Newton's laws. ... I have read, I believe, all that you 

 written on the subject, which is saying something, 

 :ig that it is scattered through seven volumes, and I 

 feel convinced with you, that the composition of velo- 

 cities and the proportionality of velocity to pressure must 

 be separately proved. What appears to me is this, that 

 Y<>ur three laws do not correspond to Newton's three 

 a, though you lead one to suppose so. My notion is 

 that Newton's third law is not included in yours ; that 

 he speaks simply of action and reaction in the ordinary, 

 accurate, and statical sense of the word. No one can, I 

 think, read his second law without seeing that it contains 

 two distinct propositions. . . . You will find, I think, that 

 all who follow Newton closely, find this double meaning 

 in this second law, and reduce the third to the literal 

 statement of action and reaction. I have no doubt that 

 but for a little love of the numbers three and seven, 

 Newton would neither have so divided his laws of motion, 

 nor his spectrum. According to my view his laws of 

 motion would have been four. 1. Inertia ; 2. Com- 

 position of Velocities; 3. Proportionality of Force to 

 Velocity; 4. Equality of Action and Reaction. The 

 :<!), like you. consider the last a necessary truth, 

 and rcdiic- tin- iirst principles to Newton's first laws; 

 do the same, but break them up into three. I am 

 <|uitc convinced that action and reaction can le 

 argued it priori satisfactorily, at least human minds are 

 no means clear upon it, and Robison well ols i 

 ' no one before Gilbert seems to have thought that a 

 magnet was attracted by iron, as much as iron by the 



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