Leslie on Meteorology. 183 
contrived by M. de Saussure, to measure the variable intensity of the eceru- ; 
lean hue, which the sky assumes in different climates and elevations, accord- 
ing to the progress of the day, or advance of the season. It consists of 
53 slips of paper, of about a quarter of an inch broad, stained with the 
successive shades of blue, from the palest sapphire to the deepest azure, 
which are pasted around the circumference of a circle of pasteboard, of 
about four inches in diameter. The colours were obtained from fine 
Prussian blue, diluting it with white chalk, or darkening it with a mixture 
of ivory black. He likewise compared those coloured spaces with the pure 
tints of a solution of copper in ammonia, which resemble most the soft 
transparent hues of the atmosphere. To represent the effect of clouds, 
and diffuse aqueous vapours, he dropped into that liquid a portion of very 
fine divided argillaceous earth, precipitated by ammonia from a solution 
of alum *. 
We think with Mr. Leslie, that “ it would be quite impossible 
to paint, with any water-colours, two cyanometers that should 
continue to agree, after being exposed for some time to the 
action of the airand the sun.” But we conceive that the fol- 
lowing arrangement would answer: A saturated solution of 
sulphate of copper, deepened by ammonia, enclosed ina glass 
vessel of a definite size, being assumed as the maximum depth 
of shade; that then the successive gradations could be accu- 
rately obtained by successive dilution of the blue liquid with 
water. Or the solution could be included between two planes 
of glass, joined together to form a wedge, the length of the side, 
and the thickness of the base of which, might be some definite 
measure. The side being graduated, and furnished with an eye- 
hole in a sliding plate of brass, the comparison of the sky- 
colour, to that of the liquid, might be accurately made. 
The 8th, 9th, and 10th sections of Mr. Leslie’s Dissertation, 
which treat of the wind-gauge, rain-gauge, and electrometer, con- 
tain nothing of particular interest. In the 11th, we have a short 
account of the drosometer, an instrument proposed in 1727, by 
Weidler, a German professor, to measure the quantity of dew 
which gathers on the surface of a body which has been exposed 
to the open air during the night. Dr. Wells, in his excellent Trea- 
tise on Dew, has furnished the ready mode of measuring the rate 
of this deposition, by the increase of weight which parcels of wool, 
exposed in different circumstances, acquire. Weidler’s instru- 
ment consisted of a bent balance, which marked in grains the 
preponderance which a piece of glass of certain dimensions, 
laid horizontally in one of the scales, had acquired from the 
settling and adhesion of the globules of moisture. Mr. Leslie’s 
proposal of spreading a ‘coat of deliqueate salt of tartar,’ 
with a hair pencil, over “ the shallow surface” of a glass fun- 
nel, and renewing this coat as often as occasion may require, to 
facilitate the descent along its sides into a graduated glass tube, 
is such a project as no practical chemist would ever think of ; 
¥* P. 350. 
