164 



DO AND DO NOT WITH SUNLIGHT 



[Cn. VI 



265. Summary of Chapter VI: 



Do Do NOT 



i . Utilize sunlight when that i . Do not use artificial light 

 is available, for it is the bright- in a region where bright sun- 

 est light to be had on our planet light is constantly available. 

 ( 232). 



2. For sunlight some sort of 

 a heliostat is necessary to 

 counterbalance the rotation of 

 the earth, and make the sun 

 shine in one place continuously 

 ( 233). 



3 . Two motions to the mirror 

 are necessary, an up and down 

 motion and a rotary motion at 

 right angles to this ( 233). 



4. If a clock-driven heliostat 

 is used, the instrument must be 

 set up so that the shaft of the 

 clock shall point toward the 

 celestial pole and thus be 

 parallel with the earth's axis 

 ( 239-241). 



5. To make the shaft parallel 

 to the earth's axis raise it from 

 the horizontal an amount equal 

 to the latitude of the place 

 where it is to be used ( 240). 



6. The two-mirror heliostat 

 is simplest and least expensive 

 ( 239). 



2. If a porte-lumiere is used 

 to keep the sun shining in one 

 place, do not forget to adjust 

 the mirror frequently. Remem- 

 ber that the earth never stops 

 rotating. 



3-4. For a clock-driven helio- 

 stat do not forget that the shaft 

 moving the mirror must point 

 toward the north pole (or 

 south pole, if south of the 

 equator) . 



5. Do not forget to elevate 

 the clock-shaft an amount equal 

 to the latitude of the place. 



6-7. Do not put the second 

 mirror of the heliostat so that 

 the sun cannot shine on the 

 first mirror. 



