Royal Society of Edinlurgh. 309 
ferent from all the other crystals of the mineral kingdom; com- 
bining in the same specimen the structure of both the classes of 
doubly refracting crystals. In one part of their mass they have 
the same structure as calcareous spar, and crystals of that class ; 
in another the structure of sulphate of lime, and crystals of that 
elass ; and in some parts they do not exhibit double refraction. 
Experiments made at Woburn Abbey for determining the Pro- 
portion between the Load and Draught of Horses in Waggons. 
The instrument employed was contrived by Mr. Salmon of 
Woburn. One general result was, that in a four-wheeled wag- 
gon of the ordinary construction, on a good road and on a hori- 
zontal plane, the draught is between a 25th and a 30th of the 
Joad. With a load of one ton the draught is between 75 and 
80 pounds. Several other results were deduced. 
Account of a Chromatic Thermometer, by Dr. Brewster. This 
instrument is founded on a newly discovered action of heat upon 
glass. Glass is thrown into a éransient. state of crystallization 
during the propagation of heat through its mass. When in this 
state it acts on polarized light like crystallized bodies, and pros 
dices various orders of colours in different parts of the glass- 
plate. The number of fringes increases, or the tints rise in 
Newton’s scale, as the temperature of the source of heat is in- 
creased ; so that the difference between the temperature of the. 
glass and that of the source of heat, is measured by the num- 
ber of the fringes, or the nature of the tints which are developed, 
As every tint in the scale of colours has an accurate numerical 
value, differences of temperature may be measured with the ut- 
most correctness, from the lowest temperature up to those at 
which glass begins to lose its solidity. 
The heat of the hand applied toa single plate of glass three- 
tenths of an inch thick, produces a perceptible effect in crystal- 
lizing the plate ; so that, if ten plates were employed, a difference 
of temperature, equal to one-tenth of that which was applied to 
the single plate, will be distinctly appreciable. 
Extracts from an unpublished Memoir of Laplace on the Ap- 
plication of the Calculation of Probabilities to Physics. The 
extracts referred particularly to the figure of the earth as de- 
duced from the vibrations of pendulums. ‘ He finds from thirty- 
seven of the best experiments on the length of the seconds pen- 
dulum in different latitudes, that the increase of gravity from the 
equator to the poles, follows the law which theory points out as 
the most simple; and concludes that the density of the earth 
must augment regularly from the surface to the centre; and 
hence he infers the original fluidity of the whole—a state, he 
adds, which nothing but excessive heat could produce, [This 
fiuidity may be allowed without admitting the cause he has as- 
U3 signed 
