1817.) during the Year 1816, 8 
menter was placed between them ; and he varied his position till 
both the sounds appeared the same. The distance between him - 
and each of the sounding objects was then measured. Some of the 
results of these experiments are rather paradoxical, or at least they 
contradict the commonly received opinions. They are as follows :— 
‘(1.) The wind has scarcely any influence on sounds at small dis- 
tances; 20 feet, for example. 
(2.) When the distance is more considerable, the sound extends 
much less against the wind than in the direction of the wind. The 
difference increases with the distance. 
From these two propositions it results, 1. That the law of the 
decrement of sound is not the same in the direction of the wind 
and in the opposite direction. 2. That the influence of the wind 
upon sound is not greater at the place where the sound is produced 
than it is during the whole course of its passage. 
(3.) Sound is heard a little better in a direction perpendicular to 
the wind than in the direction of the wind itself. 
(4.) Causes not connected with the wind, but depending upon 
the modifications of the atmosphere, have great influence on the 
facility with which sound is propagated to a distance. 
III. OPTICS, 
1. Refractive and Dispersive Powers of certain Bodies in the 
State of Liquid and Vapour.—MM. Arago and Petit have made an 
important set of experiments on the refractive power of certain 
bodies while in a liquid state, and afterwards upon the same bodies 
when converted into vapour. Supposing the Newtonian theory to 
be correct, it is natural to conclude that the refractive power of the 
same body in different states is always as its density. But from the 
experiments it appears that when a body is converted into vapour its 
refractive power diminishes at a greater rate than its density. Thus 
the refractive power of liquid carburet of sulphur, when referred to 
air, isa little greater than 3; but when referred to air in the state 
of vapour, its refractive power is only 2. The substances experi- 
mented upon were carburet of sulphur, sulphuric ether, and mu- 
riatic ether. The dispersive power of these bodies when they are 
converted into vapour diminishes at a greater rate than their refrae- 
tive power. (Ann. de Chim. et Phys. i, 1.) 
2. Remarkable Phenomenon observable in the Diffraction of 
Light.—When an opaque body is placed in a pencil of light, its 
shade is surrounded externally by certain bands of light which are 
particularly examined by Newton in the third book of his Optics. 
Luminous bands not less remarkable appear likewise within the 
shadow. These were described by Grimaldi, and afterwards by 
Maraldi and Delisle. In the year 1803 Dr. Thomas Young pub- 
lished in the Phil. Trans. a very curious experiment respecting these 
internal bands. ‘They disappear entirely provided the rays that pass 
along either of the sides of the opaque body be stopped by an 
A 2 
