OCR Text |
Show 172 EXPANSION-SMOKE. much ease and certainty as we could state the facts before our eyes. But that is an extent of knowledge which no human being can by possibility attain; and the utmost we can expect as the reward of the most careful observation is to understand what is actually before us, and make a shrewd but silent guess at what may immediately follow. In all things the past is the only mirror in which we can see the future; and if we search for knowledge of it anywhere else, we fail in our aim, and at the same time throw away the present. When the air is heated, its tendency is to spread or expand equally in all directions, upwards, downwards, and laterally; but the actual motion is in the direction of the least resistance ; and heated air ascends in the afmosphere on the very same principle that the heated lava of a submarine volcano rises through the waters of the ocean, and does not form a flat cake, or bed, at the bottom. The heated air ascends, and as it gets into air, having less resistance to its expansive force, it expands and cools, so that it at last comes to a place where it has no tendency to move unless it is acted on by a fresh cause. We can have a very tolerable notion of it in the ascent of smoke. That is really the ascent of warm air ; and it is hindered, and not promoted, by the particles of charcoal and water, and other matters, which give the colour to the smoke. A chimney often smokes the most vigorously where it does not appear to smoke at all; that is, where there is a bright clear fire, and nothing but warm air ascending; and those furnaces which have their smoke so that it is hardlv visible, send their currents of air to a much greater height than those which rain soot all over the neighbourhood. Still the visible smoke of fires is one means of ooservation by which we can get some insight into the motions of ascending currents in the air produced by heat. When they blow all in the same AND GROWTH. 177 plo~ghmen, and picking up the " animal weeds," while the ploughs are turning down the vegetable ones. All the countless races of that time of labour and ~f l.ove, both native and visitant, are busy following thmr own purpose, or rather the law of their being for they for.m no purp?se of their own, or they would sometimes commit errors of judgment as we do, but they do not. At the same time the fulfilment of the law of their being works for good to us just as the law of the being of a bushel of wheat ~orks for go?d ~o us when we cast it upon the earth and cover It with dust; and come back after a season and find ten bu~hels, nine for food, and one to cast into the earth, m the same manner and with the same hope as before. At that season of the year nature has many busy labour~rs to feed, and many young plants, and come or commg blooms, and other previous thin a-s to look aft_er, that her grand messenger the atmo:phere re qmres to be on the alert ; and as nothing in nature ever do~s that which it is not the very law and purp. ose of Its nature to do, her messenger is always in tune., and l}O~ one of her workers slackens or is palsied until It has answered the end for which the Author of nature ordained it, and the matter which has ceased to be useful in it is required for another purpose. Th?se variable winds of the spring which seem to shtft ab.out and change in their rate and their temperature, m. a mann~r absolutely capricious, are far bore unerrmg than 1f the wisest man that ever lived ad the manageme~t or them. That is proved by the ':'ery .fact of our thmkmg them capricious, which is JUSt m other words admitting that we do not unders} and the:r;n; and of course that we could not get one ~ the t~mgs done, of ourselves, or by our directions, .whw~ they are unceasingly doing for us. The most mtelhgent of us know but few of the properties |