GENERAL CONSIDERATIONS. 15 



19. FUNCTIONS OF INSTRUMENTS. 



The foregoing remarks apply to the constituent parts of instru- 

 ments, without reference to the special department of science to 

 which they belong. 



The classification of special instruments will be best under- 

 stood if we arrange those belonging to each department of science 

 according to their respective functions some of these functions 

 having instruments corresponding to them in several depart- 

 ments of science, while others are peculiar to one department. 



All the physical sciences relate to the passage of energy, under 

 its various forms, from one body to another; but Optics and 

 Acoustics are often represented as relating to the sensations of 

 sight and hearing. These two sciences, in fact, have a physiological 

 as well as a physical aspect, and therefore some parts of them 

 have less analogy with the purely dynamical sciences. 



The most important functions belonging to instruments, or 

 elements of instruments, are as follows : 



i. The Source of energy. The energy involved in the phe- 

 nomenon we are studying is not, of course, produced 

 from nothing, but it enters the. apparatus at a par- 

 ticular place, which we may call the Source. 



2. The channels or distributors of energy, which carry it to 

 the places where it is required to do work. 



3. The restraints, which prevent it from doing work when it 

 is not required. 



4. The reservoirs in which energy is stored up till it is 

 required. 



5. Apparatus for allowing superfluous energy to escape. 



6. Regulators for equalising the rate at which work is done. 



7. Indicators, or movable pieces, which are acted on by 

 the forces under investigation. 



8. Fixed scales on which the position of the indicator is 

 read off. 



