216 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK in. 



These principles explained, it is easy to understand the mechanism 

 of the three systems of heliostats those of Gambey, Silberaiann, 

 and Foucault's. The last two are represented in Figures 151 

 and 152. 



V. THE SIDEROSTAT. 



A serious inconvenience in instruments used in observatories 

 for researches in physical astronomy is, that the observer must 

 move with the eye-piece of the telescope according to the part of 

 the heavens he wishes to study, and to the motion of diurnal 

 rotation which displaces it. He is therefore subjected to very 

 inconvenient, annoying, and fatiguing positions, which are detri- 

 mental to the study of the phenomenon observed. When observa- 

 tions have to be made with the transit telescope or theodolite a 

 rectangular prism, may be used, at the back surface of which the 

 luminous rays undergo total reflection, and send the images in a con- 

 stant direction. The instrument to which this modification is applied 

 is' called a broken telescope. But this is a solution which is not appli- 

 cable to equatorials, where the axis is moved uniformly with the axis 

 of the earth, and follows the star observed, as the diurnal movement 

 causes it to change its place. To remedy this, and to avoid the 

 inconveniences referred to, Leon Foucault constructed the instru- 

 ment which has received the name ,of Siderostat, the idea of which 

 was first thrown out by Hooke. This is nothing more than a tele- 

 scope, with its optical axis invariably fixed in a horizontal position, 

 into which the image of the portion of the heavens to be observed is 

 reflected by a mirror moved by clockwork, exactly regulated to the 

 diurnal motion. A large part of the heavens can thus, at the will 

 of the observer, pass before the telescope, which remains immovable ; 

 and the observer, without any discomfort, keeps his eye to the eye- 

 piece of the instrument. The siderostat is, then, to begin with, really 

 a kind of heliostat, in which the direction of the reflected ray remains 

 constant and horizontal. 



Fig. 153 shows its arrangement very clearly. The mirror turns 

 on a horizontal axis, and is kept in position by two vertical supports 

 moving at will on a system of rollers round a vertical axis. With it 



