40 Mr Buchanan, On a solar Calorimeter used in Egypt 



of the 18th when the sun was intensely hot and the atmosphere 

 motionless. On none of these three days was the sun at any 

 time obscured by cloud. The only interference with the sun's 

 rays was by dust, with which the desert air is always charged. 

 It made itself evident by settling upon the mirror surfaces of 

 the reflectors, where it formed a dust of infinite fineness which 

 could only be seen when regarded edgewise. The results ob- 

 tained on the forenoon of the 18th are to be taken as the best. 



Figs. 1 and 2 are photographic representations of the instru- 

 ment showing its general appearance and arrangement. Fig. 3 

 gives a section of the calorimeter, and Fig. 4 a perspective view. 



Construction of the Calorimeter. Fig. 3 is a principal section 

 of the instrument by the plane which contains its axis OP and 

 that of the earth QS. The dimensions of the parts are most 

 easily specified by their projections on the axis OP and on a line 

 at right angles to it ; but as the sections at right angles to OP 

 are all circular it will be sufficient specification of the section 

 of the instrument at any point, say B, to give the distance LB on 

 the axis from one extremity L of the steam tube to B, and the 

 radii of which the projections on the axis are the point B. These 

 radii would be, in order from the axis outwards, -|, j, 3 and 

 4 inches. 



In the following table the first line contains the points on 

 the axis, the second contains their distance from L, the lower 

 extremity of the steam tube, the third contains the radius of the 

 innermost circle, the section of the steam tube, and the following 

 lines contain the radii of the other circles in ascending order. 



Points on the Axis ... L K C B A E F G H 



Distance of points from L 4 16 17 19 19J 20§ 22 22J inches 

 Radii of the circular^ i|ii|iii 



sections of which each I i i i i i M il » 



point is the centre [ 1 1 3 5 | \ 6f „ 



and the projection J 2| 4 5| 7 „ 



The measurements are given in British units, because these 

 were used in its construction. The construction of the reflector 

 will be described later. The mirrors are carried on arms of 

 sheet brass which spring from a piece of brass tube which fits 

 telescopically over the condenser tube. Their outer extremities 

 are kept in position by being fixed to a flat ring of sheet brass, 

 £-inch wide. The mirrors are made each of a band of sheet copper 

 properly shaped, and bent round until the edges abut, when they 

 are soldered. The internal conical surface is then electroplated 

 with silver, using such technical precautions as shall insure the 

 production of a bright mirror surface. 



