50 Mr Buchanan, On a solar Calorimeter used in Egypt 



The inner radius of the annular disc is 152 inches, and the ampli- 

 tude of the sector to be removed is 



360°'^- =42-3°. 



The numerical data just worked out and relating to the re- 

 flector used are collected in the following table : 



No. of Mirror 





1 

 Inner 



2 

 Middle 



3 

 Outer 





Inclination to the Axis ... 



i 



61° 51' 



45° 



34° 6' 



Inner Radius inches 





1-34 



3-0 



5-0 





E 



3-0 



5-0 



6-85 



Width of Mirror... „ 



m 



1-88 



2-83 



3-31 



Outer Radius of 1 



flat band J " 



M 



3-40 



7-07 



12-23 





M-m 



1-52 



4-24 



8-92 



Amplitude of Sector to be) 



360- M - R 

 M 



42-3° 



105-5° 



158-5° 



The condition that one of the mirrors should be inclined at an 

 angle of 45° to the axis was suggested by the fact that this is the 

 angle of greatest efficiency and by the consideration that it is an 

 angle which is familiar in mechanical workshops, and is on that 

 account perhaps more likely to be laid off accurately than another. 

 There is, however, no particular advantage in making this re- 

 striction, because in designing a series of mirrors for a reflector 

 one of them is sure to be inclined at an angle of nearly 45° and to 

 have an efficiency which is sensibly the same as if the angle 

 were 45°. 



The general problem, to construct the principal section of a 

 reflector consisting of a series of conical mirrors when the direction 

 and length of the common focal line are given and the position 

 relatively to a point on this line, of a point occupying a definite 

 position in the line of section of one of the mirrors is simple. 

 The diagram, Fig. 6, shows a construction of this kind. 



The line OP is the axis of the reflector, it is also the direction 

 of the incident rays when the instrument is in use. Make it the 



