. Frost & fire : natural engines, tool-marks & chips : with sketches taken at home and abroad by a traveller. Railway Tka Cumulus clouds which form above London are higherthan the Alps, and, like Alps, they must be seen from adistance before the (^ye can take them in. l)ut when theyare seen, under favouraltle circumstances, their movementst(!ll of upward and downward currents in air—of expansion ATMOSPHERIC FORMS. 25 and contraction in particular regions—of currents which areanalogous to those which move up and do^vn, and sideways, inboiling water. The lower edge of a distant cloud is often ne
. Frost & fire : natural engines, tool-marks & chips : with sketches taken at home and abroad by a traveller. Railway Tka Cumulus clouds which form above London are higherthan the Alps, and, like Alps, they must be seen from adistance before the (^ye can take them in. l)ut when theyare seen, under favouraltle circumstances, their movementst(!ll of upward and downward currents in air—of expansion ATMOSPHERIC FORMS. 25 and contraction in particular regions—of currents which areanalogous to those which move up and do^vn, and sideways, inboiling water. The lower edge of a distant cloud is often nearly astraight line ; it is, in fact, the outline of the under side ofpart of a dome of vapour, forming at a certain distance abovethe earths convex surface. The upper side is a heap ofgreat rolling raoimds which are constantly moving, swelling,and shrmking ; rising and falling. As warm currents of air rise through the vapour, rollingclouds expand upwards, and change from roimded domes toconical piles, and they flow over, and spread out upon thehigher layer of atmosphere through which they have been. Rising and Falling. thrust, taking the shapes of momitains. So long as the sunwarms the cloud, or the earth beneath it, the upward expand-ing motion continues. But when the sim disappears below thehorizon, the action grows less, and the movement is great boiling mass ceases to boU; and settles down intolayers of even thickness. The Ckimulus becomes a Stratus, 26 F(DKMS. or perhaps a cold wet current of air joins company witli tliecloud ; drops grow larger and heavier, and tlie whole fabrictumbles dovm as a heavy shower. Then the Cumulus is a Mmbus, and the source of a flowing stream. It is the same whether the growth and decay of suchclouds be watched from below or from above. At sea there are no mountains to interfere with windswhich blow along the surface of the water ; so clouds, if theychange their form, alter because of forces within them. Thus, o
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