Pièce 14 - James David Forbes to William Whewell

4 pp

Paris - Since JDF arrived in Edinburgh he has been at work upon the Fundamental principles of Dynamics. He has read everything WW has ever written on the subject: 'I feel convinced with you that the composition of velocities, and the proportionality of velocity to pressure, must be separately proved. What appears to me is this, that your three laws do not correspond to Newton's three laws, though you lead one to suppose they do. 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 words'. JDF believes Newton's three laws should be four: '1st. Inertia 2nd. composition of velocities 3rd Proportionality of Force to velocity 4th Equality of action and reaction. The French like you consider the last a necessary truth and reduce the first principles to Newton's two first laws: - you do the same, but break them up into three'. JDF is not convinced that action and reaction can be satisfactorily argued a priori. 'I cannot tell you what a weight is off my mind since you agreed to modify your mechanics. I am so fully convinced that no inexperienced person can undertake to write a good systematic work - that I had resolved that nothing should induce me to do it at present'. JDF has marked in pencil on the margins of WW's last edition of his mechanics, the parts he thinks should be included in the first volume of the two suggested by WW. He hopes it will be out by October and at a cheap price.