182 Analysis of Scientific Books. 
Mr. Leslie’s fourth section (misprinted fifth) is entitled The 
Atmometer. ‘The atmometer 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 rather wide glass tube, 
bearing divisions, each of them corresponding to an internal an- 
nular section, equal to a film of liquid that would cover the outer _ 
surface of the ball, to the thickness of the thousandth part of an 
inch.”? ‘It does not mark the mere dryness of the air, but it 
measures the quantity of moisture exhaled from a humid surface 
in a given time*.” We believe that very few persons ever 
employed this instrument, the functions of which are far 
better performed by suspending, at the end of a balance, a ves- 
sel of porous earthenware, containing water. Knowing the sur- 
face, temperature, and loss of weight, the problem of evaporation 
is soon resolved. A thermometer may have its ball inserted in 
the water, its stem passing through a common cork in its 
mouth. 
The photometer, or instrument which measures the degree of 
light, is a differential thermometer, having one of its balls dia- 
phanous, and the other coated with china ink, or rather blown 
of deep black enamel. We consider two delicate thermome- 
ters of equal size filled with colourless alkohol, one of which 
has a dark coloured bulb, as a much better photometer. They 
should be both enclosed in the same cylinder of glass. We 
shall, however, transcribe Mr. Leslie’s eloquent eulogium of 
his photometer. 
The photometer, placed in open air, exhibits distinctly the progress of il- 
lumination from the morning’s dawn to the full vigour of noon, and thence 
its gradual decline till evening has spread her mantle; it marks the growth 
of light from the winter solstice to the height of summer, and its subsequent 
decay, through the dusky shades of autumn ; and it enables us to compare, 
with numerical accuracy, the brightness of different countries, the brilliant 
sky of Italy, for instance, with the murky atmosphere of Holland. ....It 
may be mentioned as a curious inference, that the light emitted from the 
sun, is 12,000 times more powerful than the flame of a wax candle; or that 
if a portion of the luminous solar matter, rather less than an inch in diame- 
ter, were transported to our planet, it would throw forth a blaze of light 
equal to the effect of 12,000 candles ft. 
In the sixth section, the Zthrioscope is described : the name of 
another very delicate modification of the differential thermometer, intended 
to measure the /rigorific impressions which are showered incessantly from 
the distant sky. A more convenient instrument for measuring 
the relation between the projection of heat from the surface of 
terrestrial bodies, and the counter-radiation from the sky, is 
obtained by placing the blackened bulb of a delicate mercurial 
thermometer, in the focus of an egg-shaped silver drinking cup. 
Its indications, compared with those of another, haying its bulb 
at the side of the cup, will give good zthrioscopic intelligence. 
The Cyanometer occupies the next section. This instrument was 
* P, 346. + P. 348. 
