1816.] during the Year 1815. 3 
as we live in an atmosphere composed of four elastic fluids; or 
supposing the effect of the carbonic acid gas and the vapour of 
water to be insensible, still every sound ought to be repeated at 
least twice by the azotic and oxygen atmospheres. As this never 
happens, it is concluded that these two gases are elastic to each 
other. ‘This objection having been considered long ago, it is need- 
less to resume the subject here. 
The distance at which sounds may be heard is much greater than 
is generally imagined. Dr. Derham informs us, on the authority 
of S. Averrani, that at the siege of Messina the report of the guns 
was heard at Augusta and Syracuse, almost 100 Italian miles dis- 
tant ; and he states upon his own authority that in the naval engage- 
ment between the English and Dutch which took place in 1672 the 
report of their guns was heard upwards of 200 miles off, as far as 
Shrewsbury and Wales. (Phil. Trans. vol. xxvi. p. 2. 1708.) 
Humboldt mentions the reports of volcanoes in South America 
heard at the distance of 300 miles; and Mr. Monro, a British 
planter in Demerara, informed a friend of mine, on whose state- 
ment I can rely, that the loud explosions which took place from the 
volcano in St. Vincent’s were heard distinctly at Demerara. Now 
this is a distance which must considerably exceed 300 miles. 
IV. OPTICS. 
The various experiments that have been made by different philo- 
sophers to determine the relative quantities of light which proceed 
from luminous bodies are known, I presume, to most of my readers, 
The curious results obtained by Bouguer and Lambert, the photo- 
meter of Count Rumford, and of Professor Leslie, deserve to be 
studied and understood by all who are interested in such pursuits. 
Lampadius has lately proposed a new photometer, and he informs 
us that he has succeeded in making his instruments agree with each 
other as accurately as different thermometers do. His photometer 
consists essentially in a tube a foot long, through which he looks at 
the luminous object. At the extremity of the tube furthest from 
the eye he places thin shavings of horn till he can no longer distin- 
guish the luminous object. At first he reckoned the degree of light 
given out by the luminous body by the number of shavings of horn 
necessary to intercept it; but as instruments constructed on such a 
plan could not be comparable with each other, he fell upon the 
following method to graduate his photometer. He burns phosphorus 
in oxygen gas, and ascertains the thickness of horn shavings neces- 
sary to intercept the light ; and he contrives, by means of a screw 
and a ring, to pack these shavings always so that they shall occupy 
nearly the same space. This space he divides into 100 degrees. 
The instrument, thus graduated, serves to measure the light emitted 
by other luminous bodies. The defects of such an instrument must 
be apparent to every person. ‘The difference in the transparency 
and thickness of the horn shavings, and the difficulty of packing 
them so that they shall always occupy the same space, must render 
A 2 
