214 THE APPLICATIONS OF PHYSICAL FORCES, [BOOK HI. 



extremity a rod BO is fixed to which an inclination is given with 

 regard to the dial, equal to the declination of the sun for the day of 

 observation. This is the first condition which all the various 

 systems of heliostats have to fulfil. 



2nd. The rod OB carries the mirror; and it is connected with an 

 articulated parallelogram, Oabc, the diagonal of which, 06, coincides 



FIG. 151. -J. T. Silbmnann's heliostat. 



with the bi-sectrix of the angle SOE, that is to say with the normal 

 to the point of incidence, the fixed side 0#, of the parallelogram 

 following the direction OH given 'to the reflected beam. Such is the 

 arrangement of the heliostat devised by J. T. Silbermann, and which 

 is shown in Figure 151. 



3rd. Suppose now, OC, a rod of constant length, taking any position 



