142 MR JOHN SCOTT ON THE BURNING MIRRORS OF ARCHIMEDES. 



but if the direct rays of the sun fall on the same plane with the light reflected by 

 the combination, the intensity will be 



nkl'f cost j, 

 p + L - 



Bringing the combination nearer the plane on which the light is thrown, 

 the intensity of the n mirrors can be made equal to the above equation by 

 diminishing the value of r. 



Hence nkl'fcosi _ '/i&I'g 2 cost T , 



* • A/ — 2/2 '2\ * • 



nf\r* — r -)cos& 



As these results are independent of the absolute value of I', the equality of 

 temperature may be detected by a Fahrenheit thermometer, or any more delicate 

 means of indicating equal temperature. 



When k is accurately known, this combination, or the conical reflector, may 

 obviously be used to ascertain the intensity of the solar beams at different hours 

 of the day and different periods of the year, and will thereby furnish data for 

 estimating accurately the heat or light absorbed by the atmosphere. The light 

 lost by the solar rays in penetrating the atmosphere being known, the intensity 

 of the radiation at different parts of the solar disc may be found by (Article 6, 

 Equation 6), if a zone or segment of a parabolic reflector can be constructed having 

 a focal length of 70 or 80 feet. 



Article 10. — Prop. When tico Conical Mirrors have a common Axis, their Surfaces being 

 either perpendicular or parallel, if the rays incident on the exterior Reflector parallel to 

 the Axis meet after reflection the interior one, they will be again reflected parallel to the 

 Axis in a beam of increased intensity. 



Let AB (figs. 12 and 13) be the common axis of two conical reflectors described 

 by the revolution of the lines MN and CD about the axis AB, CD being either 

 perpendicular or parallel to MN. 



When AB (fig. 12) is directed to the centre of the sun, the rays which fall on 

 the surface described by MN will make with it the angle 



SEN = MKH = KHD = FHC = DCB , 



because DCB = SKN, therefore FH is parallel to AB (Euclid, 1-27). The intensity 

 of the finally reflected beam at H is to that incident at K as the perpen- 

 dicular distance of K from AB is to the perpendicular distance of H from 



