GENERAL CONSIDERATIONS. 5 



against the two sides of the groove, and cannot reach the bottom. 

 The instrument has thus six solid bearings, and is kept in its 

 place by its weight, without being subjected to any unnecessary 

 strain. 



Sir William Thomson, who has bestowed much attention on 

 this subject, has adopted a somewhat different arrangement in 

 some of his instruments. A triangular hole, like that formed by 

 pressing an angle of a cube into a mass of clay, is formed in the 

 base, and a V groove is cut in a direction passing through the 

 centre of the hole. The tm;ee feet of the instrument are all 

 rounded, but of different lengths. The longest stands in the 

 triangular hole, and has three bearings ; the second stands in the 

 V groove, and has two bearings ; and the third stands on the 

 horizontal plane of the base, and has one bearing. There are 

 thus six bearings in all. This method, though it does not give so 

 large a margin of stability as the method of three grooves, has this 

 advantage, that as each of the three feet is differently formed, it 

 is impossible to put any but the proper foot into the hole without 

 detecting the mistake. ^ 



5. BEARINGS OF MIRRORS. 



In mounting mirrors it is especially important to attend to 

 the number and position of their bearings, for any stress on 

 the mirror spoils its figure, and renders it useless for accurate 

 work. 



For small mirrors it is best to make one face of the mirror rest 

 against three solid bearings, and to keep it in contact with these by 

 three spring bearings placed exactly opposite to them against the 

 other face of the mirror. These will prevent any displacement of the 

 mirror out of its proper plane. The bearings against the edges of 

 the mirror, by which it is prevented from shifting in its own plane, 

 are, in the case of small mirrors, of less importance. 



When the mirror is large, as in the case of the speculum of a large 

 telescope, a greater number of bearings is required to prevent the 



