GENERAL CONSIDERATIONS. 3 



We may make a somewhat similar classification of the functions 

 of apparatus belonging to other physical sciences such as Elec- 

 tricity, Heat, Light, Sound, &c. 



3. GENERAL PRINCIPLE OF THE CONSTRUCTION OF APPARATUS. 



There are certain primary requisites, however, which are common 

 to all instruments, and which therefore are to be carefully con- 

 sidered in designing or selecting them. The fundamental principle 

 is, that the construction of the'vinstrument should be adapted to 

 the use that is to be made of it, and in particular, that" the parts 

 intended to be fixed should not be liable to become displaced ; 

 that those which ought to be movable should not stick fast ; that 

 parts which have to be observed should not be covered up or 

 kept in the dark ; and that pieces intended to have a definite form 

 should not be disfigured by warping, straining, or wearing. 



It is therefore desirable, before we enter on the classification of 

 instruments according to the phenomena with which they are 

 connected, to point out a few of the principles which must be 

 attended to in all instruments. 



Each solid piece of an instrument is intended to be either fixed 

 or movable, and to have a certain definite shape. It is acted on 

 by its own weight, and other forces, but it ought not to be sub- 

 jected to unnecessary stresses, for these not only diminish its 

 strength, but (what for scientific purposes may be much more 

 injurious) they alter its figure, and may, by their unexpected 

 changes during the course of an experiment, produce disturbance 

 or confusion in the observations we have to make. 



We have, therefore, to consider the methods of relieving the 

 pieces of an instrument from unnecessary strain, of securing for 

 the fixed parts a determinate position, and of ensuring that the 

 movable parts shall move freely, yet without shake. 



This we may do by attending to the well-known fact in kine- 

 matics "A RIGID BODY HAS Six DEGREES OF FREEDOM." 



A rigid body is one whose form 'does not vary. The pieces of 



p, 2 



