a word compounded 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 there 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 at different places, Saussure contrived the cyanometer. It consists of pieces of paper of three quarters of an inch by half an 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 fifty-one; and these 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 Géant, 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 ten it was at the darkest, and continued so till eleven; from eleven to six it became lighter by six shades; and from six to eight it became lighter by twelve shades. The colour of the sky during the night was light blue. A cyanometer was observed at Chamouni, and another at the same time at Geneva; the shades and the change of shades at the same hours were different in each, and different also from those observed on the Col du Géant. At Geneva the colour of the sky was darkest from ten to twelve, as on the Col du Géant. In the morning the colour of the sky is not much deeper on the Col du Géant than on the plain at Geneva, which shows that the air on the mountain is then as much loaded with opaque vapour as on the plain. In the evening the colour of the sky is lighter on the Col du Géant than at Geneva; the quantity of opaque vapour therefore is greater at that time of the day in the zenith of the Col du Géant than in the zenith of the plain. In the middle of the day the colour of the sky is much darker on the Col du Géant than it is on the plain, which shows that the quantity of opaque vapour is at that hour less than on the plain. Hence it appears that the mid-day sun has a great effect in rarefying the opaque vapours on high mountains, and that it does not diminish in so great a degree the quantity of opaque vapour on less elevated plains. Saussure, Description du Cyanomètre, Mém. de Turin, 1788, 1789; Journal de Phys., 1791, vol. i.; and Saussure, Voyage dans les Alpes, sect. 2009. The cyanometer was employed by Humboldt in his travels in South America.