i6o 



HELIOSTATS FOR THE MAGIC LANTERN [Cn. VI 



FIG. 85. DIAGRAM SHOWING THAT THE ELEVATION OF THE CLOCK-SHAFT 

 AT AN ANGLE EQUAL TO THE LATITUDE OF A PLACE WILL MAKE THE 

 CLOCK-SHAFT PARALLEL WITH THE EARTH'S Axis. 



EQ Equator of the earth. 



Axis Axis The earth's axis with the north pole of the earth above and the 

 south pole below. 



N P The earth's north pole. 



S P The earth's south pole. 



42.5 Latitude of Ithaca, New York, U. S. A. 



h h Horizontal lines, that is, tangents to the earth's surface at the two 

 latitudes shown (42.5 north, 30 south). 



Z Zenith. 



A A Clock-shaft elevated from the horizon an amount equal to the latitude. 

 If continued toward the equator the clock-shaft would meet the plane of the 

 equator at right angles, hence it is parallel with the earth's axis and points 

 toward the celestial poles. 



A h Latitude (42.5 north and 30 south). 



A Z Co-latitude (47.5 north, 60 south). 



255. Finally, a hcliostat constructed for the northern hemi- 

 sphere would work equally well for the southern hemisphere if it 

 were attached to the ceiling (i.e. wrong side up) instead of being on 

 a table or window-sill right side up, for this change in position would 

 make the clock-shaft rotate in the counter-clockwise direction, as. 

 seen from above. 



