Nipher — Physics During the Last Century, 107 



This they usually called *' living force," a name which was 

 later for a time restricted to the quantity MV^, 



He, however, goes on to say that ** force and the product 

 of force into the effective space are magnitudes too thoroughly 

 unlike to be by an possibility combined into a generic con- 

 ception,'* and he recommends thatthe name force be restricted 

 to one or the other of these meanings. Nevertheless it was 

 not uncommon twenty years later to read in the books of that 

 time that the unit of force was the foot-pound. 



The word pressure is still misused in this way by many 

 literary and engineering writers. It is used to denote force 

 per unit-area, and force. 



The doctrine of the Conservation of Energy grew very 

 naturally out of the discovery that a definite quantity of heat 

 is directly producable from a definite quantity of work. In 

 a qualitative way the identity of heat and molecular motion 

 had long been insisted upon. In Rohault's Natural Phi- 

 losophy, before referred to, is a chapter devoted to this sub- 

 ject. The writer says (I, 155) : — 



** We observe that two hard Bodies rubbed against one an- 

 other, do so agitate the parts of each other, as not only to 

 burn us when we touch them, but their Motion will increase 

 to such a Degree as to set each other on Fire. Thus in very 

 dry Weather, the Wheel and the Axle-tree of a chariot, when it 

 goes very quick, and in general, all sorts of Engines which 

 are made of Matter that will burn, and which move very quick, 

 are apt to take Fire. Nothing is more common than to see a 

 Wimble grow hot in boring a Hole in a hard thick Piece of 

 Wood. So likewise, if we file or sharp a Piece of Iron or Steel, 

 it will grow so hot sometimes as to lose its Temper. And a 

 Saw, which the Wood will not easily yield to, acquires a very 

 notable Heat. But nothing sooner takes Fire than a small 

 piece of Flint or of Steel, which is struck off, and put into 

 violent Motion by striking these two against each other. Now 

 in all these Instances, there is nothing added to these Bodies 

 but Motion. 



** All the Antients who have considered the greatest Part of 

 these Experiments, have asserted that Motion is the Principle 

 of Heat; which I acknowledge with them to be true; if by 



