Another matter of interest is the origin of sea waves. Undoubtedly they are due originally to the action of the wind upon the water. Whenever two layers of fluid lie in contact with each other, and one moves faster than the other, the faster-moving layer will throw the other into waves. This is seen, not only in the action of moving air or wind upon water, but even in the action of air upon air or water upon water. From the tops of high mountains we may sometimes look down upon a flat surface of cloud beneath. On one occasion the author enjoyed a curious spectacle from the summit of an Alpine peak. The climb up had been through damp and misty air, but on reaching the summit the clouds were left behind, and a canopy of blue sky and glorious sunshine were found overhead. Beneath the clouds lay closely packed like a sea of white vapour, and through this ocean of cloud the peaks of many high mountains projected and stood up like islands. The surface of this sea of white cloud, brilliantly illuminated by the sunshine, was not, however, perfectly smooth. It was tossed into cloud waves and billows by the action of currents of air blowing over its upper surface, and it had a striking resemblance to the surface of a rough sea. When such a cloud layer is not too thick, the ruffling of its upper or under surface into cloud waves may thin it away into regular cloud rolls, and these cloud rollers may then be cut up again by cross air-currents into patches, and we have the appearance known as a “mackerel sky.” Another familiar phenomenon is that known as the “ripple-mark” on wet sand. As the tide ebbs out over a smooth bank of sea-sand, it leaves the surface ploughed into regular rounded ridges and furrows, which are stationary waves on the sand. This is called the ripple-mark. It is due to the fact that the sand, when covered by the water, forms a surface which in a certain sense is fluid, being saturated and filled with water, but the movement of this bottom sand-logged water is hindered by the sand, and hence the layer of overlying water moves over it at a different speed in ebbing out, and carves it into what are virtually sand waves. Even a dry sand or snow surface may in this manner be moulded into a wave-form by the wind, and very curious effects of this kind have been noticed and described by Dr. Vaughan Cornish, who has made a great study of the science of waves.[7] The production of waves on water by means of a current of air blowing over it is easily exhibited on a small scale by blowing through an indiarubber pipe, the end of which is held near the surface of the water in a tub or tank. The exact manner in which the moving air gets a grip of the water is not quite plain, but it is clear that, if once an inequality of level is set up, the moving air has then an oblique surface against which it can press, and so increase the inequality by heaping up the water in some places, and hollowing it out in others.