REFLECTION. 
' the arc, the same as the Hadley’s quadrant, 
these are / subdivided into three, each of 
which will be twenty minutes. The ver- 
niers include fifty-nine of these divisions, 
and are divided into sixty, the coincidences 
of tliese will subdivide each original divi- 
sion of the circle into sixty parts, each 
equal to twenty seconds. The arm on which 
the vernier, D, is fixed, has a clamp at its 
end to fasten it to the circle ; and a fine 
screw, X, to move it slowly a small quantity 
after it is clamped. 
We shall now describe the manner of 
making an observation, by this instrument, 
of the angle between two objects, nearly 
in the same horizontal plane, we suppose 
all the adjustments of the instrument to be 
perfect ; the observer first holds the insti'u- 
ment in his right hand, by the handle 
screwed to the lower handle of the tube G, 
he looks tlirdugh the telescope, fc, and un- 
silvered part of the horizon glass, h, and di- 
rects it to one of the objects which will be in 
the dotted line, kg, he then turns the index 
and index glass, 6, by its arm D (which must 
be undamped) until the other object in the 
line, y b, is reflected from b to 7i, and from h 
by the silvered part of the glass, into the 
line kg, in which is placed the obsfer'ver’s 
eye ; he then clamps the arm, D, and 
gently turns the screws, x, backwards or 
forwards until the reflected image of the 
object in the line y, and the other object 
seen through the telescope, both exactly 
cover one another. The observation is now 
half made, and the observer reads olF and 
writes down the degrees, minutes, and se- 
conds, of each vernier, he then inverts the 
instrument holding it by the handle, m, and 
directs the telescope to the object, in the 
line yb, and brings the reflected image of 
the object, in the line kg, into view, by 
turning round the index and index glass the 
same as before ; the observation is then 
read otf and registered. To determine the 
angle measured, a mean of the products of 
both observations must be taken, this is the 
angle between the lines, yb and kg. A 
small microscope, M, in the plan, is used 
to examine the verniers, and it can be 
applied to either verniers as required. 
The dark glasses, op, are only wanted in 
observing the sun or moon. 
It is evident that by inverting the instru- 
ment, as we have described, the index 
error is of no consequence, as it will be al- 
ways more in one observation and less in 
the other. 
REFLECTION. As the rays of light 
are reflected by polished surfaces, so it is 
found that the rays of caloric have the 
same property. The Swedish chemist 
Scheele discovered, that the angle of reflec- 
tion of the rays of caloric is equal to the 
angle of incidence. This has been more 
fully established by Dr. Herschel. Some 
very interesting experiments were made by 
Professor Pictet of Geneva, which proved 
the same thing. 
These experiments were conducted in 
the following manner. Two concave mir- 
rors of tin, of nine inches focus, were 
placed at the distance of twelve feet two 
inches from each other. In the focus of the 
one was placed the bulb of a thermometer, 
and in that of the other a ball of iron two 
inches in diameter, which was just heated so 
as not to be visible in the dark. In the 
space of six minutes the thermometer rose 
22°. A simitar effect was produced by 
substituting a lighted candle in place of the 
ball of iron. Supposing that both the light 
and heat acted in the last experiment, he 
interposed between the two mirrors a plate 
of glass, whth the view of separating the 
rays of light from those of caloric. The 
rays of caloric were thus interrupted by the 
plate of glass, but the rays of light were not 
perceptibly diminished. In nine minutes 
the thermometer sunk 14° ; and in seven 
minutes after the glass was removed, it rose 
about 12°. He therefore justly concluded, 
that the caloric reflected by the mirror, was 
the cause of the rise of the thermometer. 
He made another experiment, substituting 
boiling water in a glass vessel ia place of the 
iron ball ; and when the apparatus was ad- 
justed, and a screen of silk which had been 
placed between the two mirrors removed, 
the thermometer rose 3° ; namely, from 
47° to 50". The experiments were varied 
by removing the tin mirrors to the distance 
of 90 inches from each other. The glass 
vessel, with boiling water, was placed in 
one focus, and a sensible thermometer in 
the other. In the middle space between 
the mirrors, there was suspended a common 
glass mirror, so that either side could be 
turned towards the glass vessel. When the 
polished side of this mirror was turned to- 
wards the glass vessel, the thermometer 
rose only five-tenths of a degree ; but when 
the other side, which was darkened, was 
turned towards the glass vessel, the thermo- 
meter rose 3° 3'. And in another experi- 
ment performed in the same way, the ther- 
mometer rose 3° when the polished side 
of the mirror was turned to the glass 
N n 2 
