176 SCIENTIFIC APPARATUS. 



The uses to which all astronomical instruments may be put 

 may be roughly separated into two large groups : 



I. They may be used to study the positions, motions, and 

 sizes of the various masses of matter in the universe. 

 Here we are studying celestial mechanics or mechanical 

 astronomy. 



II. They may be used to study the motions of the molecules 

 of which these various masses are built up, to learn their 

 quality, arrangement, and motions. Here we are study- 

 ing celestial physics, or physical astronomy. 



And the instruments may be arranged either to increase the 

 power of the eye or to secure photographic records. 



The instrument out of which the instruments comprised in the 

 first group have sprung, dates from Hipparchus (160 B.C.), in whose 

 time divided circles were first used. It consisted of two circles of 

 copper, one smaller and free to move inside the other. The larger 

 one was divided into 360, and the inner interior one carried two 

 pointers. This was placed in the plane of the meridian, and used 

 for observing the sun's altitude, and was the first meridian instru- 

 ment. This and the quadrant preferred by Ptolemy to the com- 

 plete circle, were the parents of mural circles and the transit 

 instruments used in our own day. 



It is to Hipparchus himself that we owe the first instrument by 

 which positions could be noted on any part of the celestial vault 

 "extra-meridional" observations, as they are termed. His 

 astrolabe and other instruments are the foreshadowings of the 

 Armillce alice j&quatoricz and the Armillce. Zodiacales of Tycho 

 Brahe, and of the modern altazimuths and equatorials. In the 

 Collection is a model showing, in their most simple form, the 

 principles of the instrument used by Hipparchus for determining 

 either the right ascensions and declinations, or the latitude and 

 longitude of the heavenly bodies, and which enabled him to 

 discover the precession of the equinoxes. 



