46 Description of an Almometer. 
force of the solar beams measures only 25 degrees. Between 
a fourth and fifth of the whole solar light is lost in its ver= 
tical descent to the earth’s surface, and in a hazy sky it often 
amounts to a third. We usually form false estimates of the 
guantity of indirect light reflected from the sky; in our 
climate it may amount to 30 or 40 degrees in summer, and 
10 or 15 in winter. This secondary light is most powerful 
when the sky is overspread with thin fleecy clouds; it is 
feeblest either when the rays are obstructed by a mass of 
thick congregated vapours, or when the atmosphere 1s pers 
fectly clear and of an azure tint. The photometer in the 
open air is not only affected by light from the sky, but also 
by what is reflected from surrounding objecis. This may 
be entirely prevented by sereens. This instrument fur- 
nishes a ready mode of ascertaining the various degrees of 
transparency. Of 100 parts of the whole incident light, 
eambric transmits 80, and when wetted 93; fine paper 
suffers 49 to pass through, But when oiled 80. It also en- 
ables us to ascertain the relative density of lights. An or- 
dinary wax candle placed two inches from the balls of the 
photometer gave an impression of six degrees; on drawing 
it back the effect diminished according to the square of the 
distance. At the distance of four feet, where the flame 
presented the same visual magnitude as the sun, its action 
was reduced to the 96th part of a degree. But the full im- 
pression of the solar rays, if not enfeebled ip passing the 
atmosphere, would be 125 degrees: therefore the sun’s light 
is 96 limes 195, or 12,000 times more powerful than that 
of a wax candle. Ifa portion of the luminous solar matter, 
rather less than half an inch in diameter, were transported 
to our planet, it wonld emit light equal. to 12,000 candles. ° 
The separate and distinct effects of evaporaiion, the cold- 
ness it occasions, and the quantity of moisture it abstracts, are 
rendered very obvious by the hygrometer when contrasted 
with an instrument which Professor Leslie has lately in- 
vented to measure the quantity of exhalation from a humid 
surface in a given time. This instrument* be calls an 
Atmometer (ftom arpos, exhalation or vapour, and jerpov, a 
measure) see fig. 16, Plate I. It consists of a thin ball of 
porous earthenware, two or three inches in diameter, with 
a small neck, to which is firmly cemented a long and ra- 
ther wide tube, bearing divisions, each of them correspond: 
ing to an internal annular section, equal to a film of liquid 
® Jt is made under the direction of its inventor by Mr. Cary, optician. 
that 
