498 JAMES CLERK MAXWELL. 



cate Maxwell's anxiety that definite meanings should be 

 assigned to all scientific terms, and that they should be used 

 only in the senses so defined. A further illustration of this 

 trait in his character is found in his paper " On the Mathe- 

 matical Classification of Physical Quantities," published in 

 The Proceedings of the London Mathematical Society, vol. iii. 

 No. 34. We give one other example, taken from Matter 

 and Motion, a little work published by the S. P. C. K., 

 and a model of scientific accuracy and philosophic thought. 

 " When the simultaneous values of a quantity for different 

 bodies or places are equal, the quantity is said to be 

 uniformly distributed in space." "When the successive 

 values of a quantity for successive instants of time are 

 equal, the quantity is said to be constant" Such examples 

 might be multiplied almost indefinitely from Maxwell's 

 published writings. The more elementary the work the 

 greater need did he see for exactness in definitions and 

 accuracy in the use of words ; and perhaps no one has done 

 more than Maxwell in giving accuracy to the expression of 

 scientific thought. His influence in this respect has especi- 

 ally been felt by all who have attended the Physical School 

 in Cambridge. On one occasion he remarked, in his halfr 

 humorous way, that spheres are inclusive figures but circles 

 are exclusive : people are always trying to get into other 

 circles, but to do so they must get out of their own spheres. 

 4. The keystone as well as the foundation of physical 

 science is dynamics a subject to which Maxwell was 

 greatly attracted in early days, and to which, even in 

 its most elementary principles, he constantly reverted 

 in after life. We have already referred to his Dyna- 

 mical Theory of the Electromagnetic field ; the sections of 

 his Treatise on Electricity and Magnetism, which are devoted 

 to dynamical principles and the general equations of motion, 

 form a most valuable compendium of dynamics ; while for 

 those who require less strong meat, his treatment of the sub- 

 ject in the Theory of Heat leaves little to be desired ; and 

 last, but not least, under the comprehensive title of Matter 

 and Motion, we have a treatise on dynamics written for 



