206 PETER GUTHRIE TAIT 



he had not himself in 1855 "read Newton's admirable introduction to 

 the Principia" When exactly Tait made full acquaintance with Newton's 

 dynamical foundation we cannot tell ; but, if we may judge from the following 

 sentences in his inaugural lecture at Edinburgh, it must have been during 

 1860 at the latest: 



That godlike mortal, as Halley does not scruple to call him, who, finding the very 

 laws of motion imperfectly understood, in a few years not only gave them fully and 

 accurately, and devised a mathematical method of almost unlimited power for their 

 application, but explained most of the phenomena of the Solar System including 

 Tides, Precession and Perturbations (though this is but one part of his contributions 

 to Natural Philosophy) and who was only, after repeated solicitations, persuaded 

 that he had anything worthy to offer to the world, will remain to all time the beau- 

 ideal of magnificent genius and devoted application, alike unstained by vanity and 

 unwarped by prejudice. 



In this inaugural lecture we find also an absolutely clear account 

 of the meaning of the Conservation of Energy. A few quotations will 

 make this clear. 



When we talk of the Conservation of Force as a principle in Nature, it is to be 

 carefully noted that we do not mean force in the ordinary acceptation of the word 

 and, indeed, the principle is now better known as the Conservation of Energy. As 

 this is a matter of very considerable moment I shall treat of it with a little detail. 



Energy may be Actual* or Potential. Actual Energy belongs to moving bodies 



Potential Energy belongs to a mass or a particle in virtue of its position, and is in 



general work which can be got out of it on account of that position Supposing that 



you have now an idea as to the meaning of these two terms, I give the principle of 

 the Conservation of Energy as it has been put by Professor Rankine to whom these 

 terms are due 



"In any system of bodies, the sum of the potential and actual energies of the 

 bodies is never altered by their mutual action." 



It is abundantly evident that before Tait entered on his Edinburgh 

 career he had already thought deeply on the new doctrine of energy. When 

 the time came for the second edition of Dynamics of a Particle, the founda- 

 tions of the subject were at once brought into line with these modern views, 

 although in most other respects the book did not materially change its 

 character. It is amusing to read Tail's own pencilled annotation on the 

 first page of the preface to the second edition, "Very poor book others 

 poorer." 



As edition after edition was called for, Tait often felt a strong inclination 

 to recast the whole work. Lack of the necessary leisure combined with 

 1 The term Kinetic was not invented till two years later. 



