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POPULAR SCIENCE REVIEW. 
theoretical amount for half the distance, i.e ., four times, which 
would be 0*001776. 
An observation on the “weight of sunlight” is thus obtained- 
On a dull day in December it is found equal to 10-2 candles, six 
inches off. This is by calculation equivalent to 2-3 tons per 
square mile. 
Leaving at this point, according to the chronological order 
adopted, communications to the Eoyal Society, an important 
paper is contributed to the “ Philosophical Magazine ” by Mr. Gr. 
Johnstone Stoney. It commences by reciting the measurements 
just given, with others contained in an article on “weighing” 
a beam of light, contributed by Mr. Crookes to the periodical 
“ Engineering” of February 18, 1876. The pressure is computed* 
as equal to the hundredth of a milligramme on the square 
centimetre. The object of Mr. Stoney’s communication is “to 
show that an excess of pressure of about this amount will arise 
under the operation of known laws ” 
He begins by estimating the outstanding pressure in the 
chamber of the instrument as about -JL- millimetre of mer- 
cury, caused in part by atmospheric air, in part by vapours 
of mercury and hydrocarbons, with, perhaps, other substances. 
Upon the vane fall such radiations from the candle as can pass 
through glass, which are competent to heat the blackened disc 
but not the transparent glass. It is assumed that the disc is 
thus heated Centig. more than the glass, and that it in turn 
warms a layer of air in contact with it. Such a layer, in 
ordinary air, would be thin, about one “ fourth metre,” or the 
thickness of a sheet of paper, for 20° Centig.; and for 0*1 half 
a “ sixth metre,” explained as being a quantity equal to the 
wave length of light of mean refrangibility, and ^th the size 
of human blood globules. Now, in the vacuum chamber the 
density is assumed at of an atmosphere, and the thickness 
of the heated layer in this attenuated medium is 10,000-f 
times greater, or more than a decimetre, thus able to reach to 
the walls of the globe. “ Some of the additional momentum 
communicated to molecules of air by the heated disc, instead 
of expending itself in intra-aerial collisions, and thus increasing 
the general temperature and pressure of the air, makes its way 
across the intervening stratum to the opposite walls of glass, 
where it occasions an increased pressure against them, of which 
the result is directed perpendicularly from the disc. An excess 
of force, equal and opposite to that on the glass, acts against 
the front of the disc, and is sufficient to account for the pheno- 
mena Mr. Crookes has investigated. The molecules reach the 
disc with velocities corresponding to 15°, and are thrown off 
from it with velocities corresponding to 15°*1. The augmenta- 
tion of pressure will be half what would arise if they had’ 
