2-12 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK in. 



supplement of this angle we have that of the two faces of the 

 crystal. 



Fig. 149 shows one of Babinet's goniometers mounted on a 

 tripod stand, but smaller ones are made which may be held in the 

 hand. The same instruments may be used in optical researches when 

 prisms are employed, the angles of which require to be known with 

 accuracy. 



IV. THE HELIOSTAT AND SIDEROSTAT. 



In many optical experiments, it is necessary to project, in a constant 

 direction for some time a beam of solar light, which, without certain 

 precautions, the diurnal motion of the sun would render impossible. 

 If the beam is first received on a plane mirror, whence it is again sent 

 back, by reflection, towards the point or towards the object to be 

 lighted up, the inclination of the mirror must constantly change 

 in order to give a constant direction to the reflected beam. 



This is accomplished by means of an arrangement which was 

 described when speaking of the solar microscope (Forces of Nature}. 

 This consists of a mirror capable of turning at will round two axes, 

 one horizontal the other vertical if the reflected beam itself takes a 

 horizontal direction. But the assistance of the observer is always 

 necessary to regulate the orientation of the mirror in a proper 

 direction. 



Heliostats are instruments intended to give this assistance ; the 

 mirror which forms the reflecting portion is put into continuous 

 motion by clockwork, and a suitable mechanism keeps it at such an 

 inclination that the solar rays, reflected on its surface, take a constant 

 direction, in spite of the diurnal movement of that body. 



-There are heliostats of various kinds dating from the time of 

 s'Gravesande, but we will mention those only named from their 

 inventors, Silbermann and Foucault. But in the first place, we will 

 point out the principles common to all, without which neither 

 their arrangements nor the working of their mechanism can be well 

 understood. 



The line PP' (Fig. 150) representing the axis of the earth the in- 

 variable line around which the diurnal movement of the stars and sun 

 takes place, the circle S will be the apparent path traversed by this 



