148 



HELIOSTATS FOR THE MAGIC LANTERN [CH. VI 



FIG. 78. ONE-MIRROR HELIOSTAT OF FOUCAULT. 

 (From his Recueil des Travaux Scientifiques). 



Modified by extending the clock-shaft above and below. 



B Clock-work. 



T The clock-shaft. 



C The upper end of the clock-shaft. 



N P Continuation of the clock-shaft above the mirror (M). 



L The angle above the horizontal to make the clock-shaft parallel with the 

 earth's axis. It equals the latitude of the place where the heliostat is used (see 

 fig- 85). 



D Divided semicircle to enable one to set the instrument according to the 

 sun's declination. 



N 1 N, N C l Connections between the clock-work and the mirror. 



clock-shaft is raised above the level 42.5 degrees. If now one were 

 to sight along the clock-shaft it would be found pointing directly 

 toward the north star. 



As seen from the diagram, fig. 85, it will then also be parallel 

 with the earth's axis. 



