6 SCIENTIFIC APPARATUS. 



mirror from becoming strained by its own weight ; but in all cases 

 the number of fixed bearings at the back of the mirror must be 

 three and only three, otherwise any warping of the framework will 

 entirely spoil the figure of the surface. 



6. BEARINGS OF STANDARDS OF LENGTH. 



It is of the greatest importance that the standard measure of 

 length, by which the national unit of length is defined, should not 

 be exposed to strain. 



The box in which the standard yard is kept in the Exchequer 

 Chamber is provided with bearings, the positions of which have 

 been arranged so that the bar may rest on them with as little 

 strain throughout its substance as is consistent with the fact that 

 it is a heavy body. 



7. ON THE BEARINGS OF MOVABLE PARTS. 



The most important kinds of motion with one degree of freedom 

 are, (i) Rotation round an axis; (2) Motion of translation with- 

 out rotation ; and (3) Screw motion, in which a definite rotation 

 about an axis corresponds to a definite motion of translation along 

 that axis. 



For one degree of freedom five solid bearings are required, the 

 sixth condition being supplied by that part of the instrument 

 which regulates the motion of the piece. 



The construction of pieces capable of rotation about an axis is 

 better understood than any other department of mechanism. 



In astronomical instruments, four of the bearings are supplied 

 by the two Y's on which the cylindrical end-pieces of the axle 

 rest, and the fifth by the longitudinal pressure of a bearing 

 against one end of the axle, or a shoulder formed upon it. The 

 weight of the instrument is generally sufficient to keep it in 

 contact with its bearings ; but when the weight is so great that 

 the pressure on the bearings is likely to injure them, the greater 

 part of the weight is supported by auxiliary bearings, the pressure 



