CONCERNING SCIENTIFIC APPARATUS. 507 



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 subjected 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 kinematics " A 

 EIGID BODY HAS Six DEGREES OF FREEDOM." 



A rigid body is one whose form does not vary. The pieces of our instru- 

 ments are solid, but not rigid. They are liable to change of form under stress, 

 but such change of form is not desirable, except in certain special parts, such 

 as springs. 



Hence, if a solid piece is constrained in more than six ways it will be 

 subject to internal stress, and will become strained or distorted, and this in 

 a manner which, without the most exact micrometrical measurements, it would 

 be impossible to specify. 



In instruments which are exposed to rough usage it may sometimes be 

 advisable to secure a piece from becoming loose, even at the risk of straining 

 and jamming it ; but in apparatus for accurate work it is essential that the 

 bearings of every piece should be properly defined, both in number and in position. 



4. METHODS OF PLACING AN INSTRUMENT IN A DEFINITE POSITION. 



When an instrument is intended to stand in a definite position on a fixed 

 base it must have six bearings, so arranged that if one of the bearings were 



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