Their actual altitudes are very variable, being greater in summer than in winter, and reaching a maximum for any given station after a long spell of hot weather. Exact measurements have not yet been made in tropical latitudes or in polar regions, but there is every reason to expect that the upper limit of cirrus for equatorial districts will be found to be much higher than in the temperate zones where actual observations have been made. In places nearer to the Arctic Circle it is also almost certain that the altitudes will be less. In the New England states, as shown by the Blue Hill observations, the maximum altitude for summer was found to be little under 15,000 metres. At Upsala, in Sweden, it was 13,300 metres. The average altitudes at the same observatories were, respectively, about 9900 and 8800 metres. At Exeter the writer’s own measurements give an average for the summer months of 10,200 metres, with a minimum rather lower than was the case in America or Sweden, and with a maximum far above the foreign values. In winter cirrus certainly comes much lower down, but the number of observations is fewer. [Illustration: Plate 5. HIGH CIRRUS. (_Cirrus Excelsus._)] The loftiest variety of cirrus appears in the afternoon in very hot weather, sometimes quite late in the evening; and in autumn it is by no means a rare event for it to suddenly form just when the sunset colours are fading, or even after they have paled into twilight. Under such circumstances it stands out of a shining silvery grey colour against the background of the darkening sky. A specimen of it is shown in Plate 5, which shows its extreme slightness of form and delicacy of texture. Sometimes it remains visible so long after the stars have begun to show as to give the idea that it is self luminous, and the illusion is certainly very strong. The writer has noted several instances in which it was plainly visible, like a silvery curtain, though the sky as a whole was so dark that stars like the five brightest points of the Great Bear could be seen through the cloud, and much smaller stars down to the third and fourth magnitude were plainly visible in the clear intervals. It has sometimes been called luminous cloud, and Mr. Ley estimated its altitude at upwards of 90,000 metres; but if we think of it as reflecting the light of the distant colourless twilight there is no need to regard it as anything fundamentally different from other clouds, or to assume a greater altitude than we know to have been the case. The specimen figured occurred in the early afternoon on June 12, 1899, at Exeter, and careful measurements of its altitude were made. This worked out as 17·02 miles, or more than 27,000 metres, a value so much greater than all other measurements of the kind that it was only after most careful verification and reference to duplicate records that it could be accepted. It differs in several ways from the lower varieties, being thinner, more glistening, and in every way more