CHAP, i.] MIRRORS AND REFLECTING INSTRUMENTS. 213 



latter body in a day, the angle S OP being the polar distance of the 

 sun at the time under consideration. At A, is an equatorial dial on 

 which the shadow of the style AO marks the hour every instant of 

 the day. The line SOB then indicates the path followed by a solar 

 beam, and if we imagine that the line OB turns round the point 

 O constantly following the extremity B of the ray AB, it will be 

 the path of the incident light during the whole day. 



Suppose KB, the direction in which the solar rays are required to 

 be constantly reflected, the bi-sectrix NN T/ of the angle S OR will be 

 normal to the point of incidence, this determines the position the 



FIG. 150. Geometric principle of the various systems of heliostats. 



mirror mm ought to occupy at the supposed moment, in order that the 

 reflection be made in the desired direction. 



The whole question is, then, to keep the mirror in a position rela- 

 tively always the same with respect to the constant direction of the 

 reflected ray and to the variable direction of the incident ray. This 

 may be accomplished in many ways. 



1st. Underneath the equatorial dial is placed a clock, which moves 

 a needle AB and causes it to describe an entire circle in twenty -four 

 hours. This needle is then always placed in the direction which 

 would be exactly occupied by the shadow of the style. At its 



