BUR 
BUR 
we shall content ourselves with taking no- 
tice of the experiment made by M. Pictet. 
Two concave mirrors being placed at the 
distance of lOi feet from each other, a 
very delicate air thermometer was put into 
one of the foci, and a glass matrass full of 
snow in the other. The thermometer sunk 
several degrees, and rose again when the 
matrass was removed. When nitric acid 
was poured upon the snow (which increased 
the cold) the thermometer sunk 5° or 6“ 
lower. Here cold seems to have been 
emitted by the snow, and reflected by the 
mirrors to the thermometer, which it is 
thought could not happen unless cold were 
a substance. It has been found that upon 
an admixture of equal quantities of snow, 
w'hich is always at 32°, and of water heated 
to 172", the result is that the compound 
only retains the lowest heat of 32", so that 
140° of heat or caloric disappears. Much 
has been said respecting the point or de- 
gree at which the thermometer should indi- 
cate the presenee of heat. The e.xperi- 
ments of Dr. Crawford seem to place it at 
1268° below the present 0 ; Mr. Kirwan 
places it at 1048" ; Messrs. Lavoisier and 
La Place at 2736° ; and by a mixture of 
four parts of sulphuric acid witji three pints 
of water, it seems that it should be placed 
at 5803° below 0. Experiments of this 
kind may be made ad infinitum, and in 
time it may possibly be ascertained that 
cold is a real substance; but for (he pur- 
pose of getting an answer to the present 
question, we will accommodate the scale of 
Fahrenheit, by adding 108° thereto, so as to 
make the 0 correspond w'ith the caloric 
imbibed by snow or ice before it can 
melt. 
The superficies of spherical bodies are to 
each other as the squares of their respective 
diameters. The diameter of the moon is 
considered to be 2180 miles, and its mean 
distance from the earth 240,000 ; from 
which it follows, on the supposition that all 
tlie solar rays received by the moon were 
reflected back, and that the earth was ab- 
solutely without heat, that the effect of 
this reflection would be found to be .00367 
of a degree (for 240,000 X 2 : 178" :: 2180" : 
.00367); W'hich multiplied into 1056.25, 
and this sum increased four times for the 
increased power of the second lens, would 
give 15.51234" as the heat of the focus ; 
92.28766" below the present 0, or 124.28766" 
below the fi eezing point. This dissertation 
is interesting in another point of view, for 
this calculation ascertains that the light 
afforded by the moon, when compared with 
that by tlie sun, abtracting all impediments 
in both cases, is only as 1 to 43480. 
A subscription w'as proposed for raising 
the sum of 700 guineas, towards indemni- 
fying the charges of the inventor, and re- 
taining the very curious and useful machine 
above deseribed in our own country; but 
from the failures of the subscription, and 
some other concurring circumstances, Mr. 
Parker was induced to dispose of it to 
Capt. Mackintosh, who accompanied Lord 
Macartney in the embassy to China ; and 
it was' left, much to the regret of philoso- 
phers in Europe, at Pekin ; where it re- 
mains in the hands of persons, w'ho most 
probably know neither its value nor use. 
BuRNiNG moM«f«i/(s, the same with vol- 
canos. See Volcano. 
BURNISHER, a round polished piece 
of steel, serving to smooth and give a lustre 
to metals. 
Of these there are different kinds of dif- 
ferent figures, straight, crooked, &c. Half 
burnishers are used to solder silver as well 
as to give a lustre. 
BURNISHING, the art of smoothing or 
polishing a melaliine body by a brisk rub- 
bing of it with a burnisher. 
Book-binders burnish the edges of their 
books by rubbing them with a dog’s tooth. 
Gold and silver are burnished by rubbing 
them W'ith a wolf’s tootli, or by the bloody 
stone, or by tripoli, a piece of white wood, 
emery, and the like. Deer are said to bur- 
nish their heads by rubbing off a downy 
white skin from their boms against a tree. 
BURR fump, or Bildge pump, differs 
from the common pump in having a staff 6, 
7, or 8 feet long, with a bar of wood where- 
to the leather is nailed, and this serves in- 
stead of a box. So two men standing over 
the pump thrust down this staff, to the mid- 
dle whereof is fastened a rope for 6, 8, or 
10 to hale by, thus pulling it up and down. 
BURSARIA, in natural history, a genus 
of worms of the order Infusoria. Worm 
very simple, membranaceous, hollow. There 
are three species, viz. tlie truncatella, liiriin- 
dinella, and duplella, found in marshy water : 
the first has a white body, oval, with a large 
hollow descending to the base, with some- 
times four or five eggs at the bottom ; the 
second is a pellucid hollow membrane, mov- 
ing forwards like a bird in flight ; the third 
is found among duck-weed, without visible 
intestines. 
BURSARS, in the Scotch universities, 
are youths chosen as exhibitioners, and 
