a word composed from the Greek, signifying a measure of the intensity of blue. On high mountains the sky appears of a deeper blue than on the plain, because the air contains a smaller proportion of opaque vapour. In order to have a comparable specimen of the shade of blue of the sky, at different times and in different places, Saussure contrived the cyanometer. It consists of pieces of Constrap paper, of three-quarters of an inch by one-half inch, each coloured with a different shade of Prussian blue. The piece number 1, is coloured of a very light shade, differing in so small a degree from white that it is not distinguishable from white at a distance, at which a black circular spot of one line and three quarters in diameter becomes invisible. The piece number 2, is of a shade of blue a little darker than number 1, so that it is not to be distinguished from number 1, when viewed from the distance before determined by the black spot. The other pieces are coloured with shades of blue successively darker, each being undistinguishable from that which precedes it, when at the distance ascertained by the black spot. The darkest piece differs little from black, and is not to be distinguished from black at the ascertained distance. The series begins with a white piece and ends with a black one; including these two the whole number of pieces is 51. These 51 pieces are pasted close to each other, and in succession from the lightest to the darkest, round the border of a circular piece of pasteboard.

The colour of the sky is ascertained by holding up this pasteboard, and comparing it with the sky in an open place. If this comparison were made from a window the coloured pasteboard would not receive all the light from the sky, but would be illuminated in part by the light reflected from the building. The direct light of the sun should not fall on the cyanometer, as the instrument ought to be used in similar circumstances, and the direct light of the sun cannot always be had.

The colour of the sky depends on the quantity of opaque vapour in the air: the less vapour there is the darker is the colour of the sky. If the air were entirely free from opaque vapour, the sky would appear black. The particles of opaque vapour in the air reflect chiefly the blue rays, and thus give rise to the blue colour of the sky. The colour of the sky at the zenith is darker than at the horizon, because the quantity of vapour through which the eye looks at the horizon, is greater than at the zenith.

Saussure observed, that on the summit of the Col du Geant, from four in the morning to six, the colour of the sky at the zenith became darker by eleven shades. During the next four hours it became darker by four shades; at 10 it was at the darkest, and continued so till 11; from 11 to 6 it became lighter by 6 shades; and from 6 to 8, it became lighter by 12 shades; the colour of the sky during the night was light blue. A cyanometer was observed at the same time at Chamouni, and another at Geneva; the shades and the change of shades at the same hours were different in each, and different from those observed on the Col du Geant. At Geneva the colour of the sky was darkest from 10 to 12, as on the Col du Geant. In the morning the colour of the sky is not much deeper on the Col du Geant than on the plain at Geneva, which shows