46 Mr Buchanan , On a solar Calorimeter used in Egypt 
The Equatorial Mounting is shown in Fig. 4. The main or 
central part is a piece of stout brass tube M, 4 inches in diameter 
and 18 inches long. It is supported on an iron tripod NN by 
an iron ring R in three segments which are pinched together by 
screws SS. From the top edge of the tube for six inches downwards, 
a slot T is cut of rather over an inch clearance, and a collar V con- 
sisting of a length of 2 inches of tube that telescopes over the 
central tube, slips up and down it and can be clamped in any posi- 
tion. The slot is intended to receive the polar axis JS when adjusted 
for latitude, the collar being clamped in the right place and the 
weight X, which is much heavier than the calorimeter, keeps the 
polar axis resting firmly on the collar. The polar axis which is a 
tube of 1 inch diameter is pivoted round a horizontal axis Y which 
is also a piece of brass tube, working in bearings on the top of the 
central tube When the tripod has been set up so that the central 
tube is vertical the polar axis is adjusted for latitude by a quadrant 
or protractor, or if the pole-star is available, it is brought to be 
visible through the tube which forms the polar axis. The calori- 
meter is held by a collar W which surrounds it and is clamped 
on the condenser tube. This collar is attached to a ball and socket 
joint J which is carried by a piece of tube which fits telescopically 
into the polar axis. The ball and socket joint was found to be 
the simplest means of giving a motion for adjusting the calorimeter 
for declination. We have thus a form of equatorial mounting 
which is simple, effective, and cheap, as it is almost entirely 
made out of brass tubes. 
Construction of the Reflector . In designing the reflector actually 
used, the following determining conditions were adopted. — Length 
of focal line to be 2 inches ; angle of the middle mirror to be 45° 
and its upper rim to have a radius of 5 inches. With these data 
the 45° mirror can be completed at once and the specifications of 
the outer and inner mirrors follow by a simple graphical construc- 
tion. The physical principle involved is that the angles at which 
rays strike and are reflected from a mirror surface are equal. 
The construction is shown in the diagram Fig. 5. On the straight 
line OP which represents the axis of the instrument, lay off the 
length AB = 2 inches ; this is the focal line. Through A and at 
right angles to .4# draw Ax , and on it make AB 3 = o inches. 
Through B draw BB 2 parallel to AB 3 and make BB 2 = 3 inches. 
Join B 2 B 3 . B 2 B 3 is obviously the line representing the principal 
section of the mirror inclined at 45° to the axis. Its length 
B 2 B 3 = 8 = 2 ’83 inches. If the line B 2 B 3 be continued until it 
cuts AB produced in B 2 , then B 3 B 2 is the generating line of the 
complete cone of which the 45° mirror is a portion. Its length is 
obviously V50 = 7’07 inches. The flat band which, when bent 
