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 
0 
4 
16 
17 
19 
191 
201 
22 
22| inches 
Radii of the circular \ 
i * 
■ i 
8 
i 
8 
i 
8 
i 
8 
i 
8 
i 
8 
i 
8 
jj 
sections of which each ( 
I 
1 
4 
1 
4 
1 
4 
1 
4 
1 5 
IV2 
1 5 
jj 
point is the centre 
1 
1 
3 
5 
1 
2 
i 
6J 
and the projection J 
1 
2f 
4 
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. 
