When cumulus clouds are formed in air which is steadily moving as a whole, that is to say, when there is a steady breeze, they have a very decided tendency to follow each other in long lines. It may often be noticed that in a particular place with a certain direction of wind these long processions follow definite tracks in relation to the geographical features. The phenomenon does not seem to have been recorded except in hilly country, but has frequently been observed by the writer. It is not the same thing as the formation of stationary belts of cloud transverse to the wind. These cumulus float along with the movement of the air, and the question to be answered is, why should they follow each other so persistently, and why should the intervening belts of sky be so continuously free from cloud. If we consider that the warm damp air which supplies them is drawn from the ground, it seems that any cause which tends to direct this warm stratum into definite channels, as it is carried on by the wind, will be a competent cause of the whole phenomenon. This we find in the presence of lofty hills which stand in the way of the warm surface winds, causing them to follow more or less the general trend of the valleys, and so delivering the rising convection currents of cloud-producing air at the same spot. It is easy to conceive that other causes, such as a difference in temperature or dampness of neighbouring tracts, resulting from whether they are bare or wooded, marshland or sandy plain, might equally suffice; or might, at least, powerfully co-operate with, or counteract, the effect of hill and vale. But in any case it is plain that the geographical conditions to the windward of the place of observation not only may affect the occurrence and distribution of cloud, but if the wind is steady it is difficult to see how they could avoid affecting it.