Measurements by one or the other of the above methods have been made at several places, the most extensive series being those which have been compiled at Upsala, and at the Blue Hill Observatory in Massachusetts. The method employed by the writer at Exeter has been rather different, and a description will be found later on in the chapter on Cloud Altitudes, the fuller consideration of which comes naturally after the different forms have been described and compared. FOOTNOTE: [1] See reference No. 2 on p. 181. CHAPTER II CIRRUS A CLOUD is sometimes defined as any visible mass composed of small particles of ice or water suspended in the air, and formed by condensation from the state of vapour. As a general rule this is exact enough, but under certain circumstances it is possible to have the particles so small, and so thinly scattered, that it is not fully satisfied. The resulting mass may not be actually visible. The presence of the condensed particles may be indicated by nothing more than a slight whitening of the blue sky, or by the formation about the sun or moon of bright circles of light known as halos. If such a halo appears, it is generally a phenomenon of brief duration. Sometimes the circle breaks and becomes incomplete by the passing away of the thin patch of cloud, sometimes the cloud increases in density until the rings are destroyed. The thinnest variety of this halo-producing structure is quite invisible to the eye. It is so thin as to have no distinctly noticeable effect upon the colour of the sky, but the optical results of its presence may be very remarkable. Highly complicated systems of rings are sometimes produced, the rings, as a rule, falling into two groups. The commonest form has the sun (or moon) in the centre, and a circle of pale light at a distance of about 22 degrees. Larger rings are seen less frequently, which have an angular radius of about 46 degrees, and as a rule have the sun situated on the ring itself. In Plate 1 we have a part of such a great halo. The camera was directed towards the east, and tilted upwards at an angle of about 40 degrees. The sun was behind the camera, in the south-west, and the ring could be traced right up to it on either side. At the same time the sun was surrounded partially by a halo of the more ordinary type, which was brightly coloured, making an effective contrast to the dull white of the greater ring. The phenomenon did not last more than half an hour, and the changes in its appearance coincided with a growing density of cloud. When first noticed the great ring was alone, and the sky was of a full blue, but a silvery film came gradually up from the south-west, and the smaller and brighter halo flashed out as the delicate curtain came near the sun. Slowly the cloud spread to the north-east, gathering density from the opposite point of the compass; and by the time the ordinary halo was at its best, the great white ring had completely vanished. [Illustration: Plate 1.