

weet 
eae 
2 MR B. STEWART ON RADIANT HEAT. 
The following arrangement was adopted for the great mass of the experi- 
ments :— 
A. Is the sentient pile, with a polished 
brass cone attached to it, for col- ~ 
lecting the rays of heat. 
B. Is the galvanometer, the position 
of its needle being read to 75th 
of a degree by the telescope C. 
D, Is ascreen placed before the mouth 
of the cone in which there is a 
small hole or diaphragm ‘65 inch 
square. The screen is covered 
with gilt paper, in order that, 
should it get slightly heated, it 
might radiate as little as pos- 
sible, 

The heated body is placed behind the diaphragm, filling up the field of view 
from the cone; so that every ray reaching the cone from behind the diaphragm 
comes from the heated body. 
In the following experiments, unless the contrary is mentioned, the distance 
of the diaphragm from the mouth of the cone is 2 inches. 
The dimensions of the cone itself are as follows :— 
Length of axis, or distance between centre of mouth and pile, . 3 5 inches, 
Diameter of mouth or opening, . i si : : : 3 2°6 inches. 
The temperature to which the heated body was raised was generally 212°, 
and the apparatus used for heating it was of the following construction :— 
It consisted of a tin vessel, having its top, bot- 
tom, and sides double (or a box within a box), 
and furnished on the top with a lid, also double, 
by means of which the body to be heated was 
introduced into the interior. Water was poured 
into the chamber between the outer and inner 
boxes, and allowed to boil; and, when the lid was 
shut, the temperature of the interior was found to rise very nearly to the boiling 
point ; a thermometer placed in the air of the chamber showing a temperature of 
200°, and when lying on the bottom, a temperature of 210°. When an observa- 
tion was to be made, the hot body was taken out, and that surface which lay on 
the bottom of the inner chamber placed behind the diaphragm, so as to radiate 
into the cone. In the following experiments, unless the contrary is mentioned, 
the body has been heated in this manner. 
The first swing of the galvanometer needle was taken as representing the in- 
tensity of the heating effect; and Professor Forses has shown, in a paper read 
before this Society, 2d May 1836, that this will hold up to angles of about 20°, 
which is the maximum deviation used in these experiments. 

