INSTRUMENTS ILLUSTRATING KINEMATICS. 61 



each hour ; and this would be the second part of the descrip- 

 tion. The first part is what was called Kinematics by Am- 

 pere; it tells us how the motions of the different parts of a 

 machine depend on each other : n consequence of the machinery 

 which connects them. This is clearly an application of geometry 

 alone, and requires no more measurements than those which 

 belong to geometry, namely, measurements of lines and angles. 

 But the name Kinematics is now conveniently made to include 

 the second part also of the description of motion when and how 

 fast it takes place. This requires in addition the measurement of 

 time, with which geometry has nothing to do. The word Kine- 

 matic is derived from the Greek kinema, " motion ;" and will there- 

 fore serve equally well to bear the restricted sense given it by 

 Ampere, and the more comprehensive sense in which it is now 

 used. And since the principles of this science are those which 

 guide the construction not only of scientific apparatus, but of all 

 instruments and machines, it may be advisable to describe in some 

 detail the chief topics with which it deals. 



Dynamics. That part of the science which tells us about the cir- 

 cumstances under which particular motions take place is called 

 Dynamics. It is found that the change of motion in a body 

 depends on the position and state of surrounding bodies, accord- 

 ing to certain simple laws ; when considered as so depending on 

 surrounding bodies, the rate of change in the quantity of motion 

 is called force. Hence the name Dynamic, from the Greek dynamis, 

 "force." The word force is here used in a technical sense, peculiar 

 to the science of motion ; the connection of this meaning with the 

 meaning which the word has in ordinary discourse will be explained 

 further on. 



statics and Dynamics are again divided into two branches; the 

 Kmetics. s tudy of those circumstances in which it is possible for a 

 body to remain at rest is called Statics, and the study of the cir- 

 cumstances of actual motion is called Kinetics. The simplest part 

 of Statics, the doctrine of the Lever, was successfully studied 



