CONCERNING SCIENTIFIC APPARATUS. 517 



The classification of special instruments will be best understood 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 departments 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 : 



1. The Source of energy. The energy involved in the phenomenon we 



are studying is not, of course, produced from nothing, but it enters 

 the apparatus at a particular 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. 



Thus in solid machinery we have 



1. Prime movers. 



2. Trains of mechanism. 



3. Fixed framework. 



4. Fly-wheels, springs, weights raised. 



5. Friction breaks. 



6. Governors, pendulums, balance springs in watches. 



7. Dynamometers, Strophometers, Watt's indicator, chronographs, &c. 



8. Scales for these indicators. Standards of length and mass. Astro- 



nomical standard of time. 



