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Show 324 EVILS OF MOSS AND LICHEN. ceeded, it becomes an evil. For, though their tend· ency be to mitigate the severity of both heat ~nd cold, they do in all cases produce cold b¥ makmg the total evaporation greater than it otherw1s~ would be. They take off the extremes of evaporatwn during the great heats, but they also occasion evap?ration at times when otherwise there would be little or none, and thus they keep a moist atmosphere all the year round, and so where t~1ey abound the climate is less healthy. It also rams much more frequently, because tha_t air, being a~ways nearly sa~urated with moisture, 1s of course d1sposed to part w1th that moisture in the state of rain, much more readily, that is, with much less atmospheric action, than when the degree of saturation is less. Consequently they are injurious to cultivated grounds. To the annual crops they, indeed, do small harm, as they attain but little size, and are under the shade of these. But on grass lands they are much more destructive ; and would in time change a good soil and climate into the opposite. The finest grasses, thourrh they thrive well with occasional irrigations, decay when they are too moist, as they al_ways are in old pastures that. have got mossed; and 1f ~he _surface had little dramage, the mosses would, m time, dislodo·e all the grasses, and produce a surface not well :dapted for any kind of culture. When land has once come to that state, the only, or at least the effectual means of arresting the mischief, are the spreading of alkaline substances, trenching, or paring oif and burning the sod. But it would be endless to enumerate even the trains of speculation and inquiry that present themselves to any one who studies vegetables, in their connexion and succession, however narrow the field of observation may be. A step taken anywhere that there are plants furnishes a study; and that walk which does not afford reflection for a week must be very short, as well as over a place compaxatively INTEREST O.F A WALK. 325 barren. Even a public road may answer the purpose, for there are the hedges with their wild plants creeping below or entwined among the bushes· and as the hedge is a sort of hill, and the ditch a s~rt of valley, the two together form a sort of epitome of a cons1derabl~ tract ?f country. The changes that take place_ m the wild plants, from changes of soil and elevatwn, present a constant succession of new objects, so that, upon the most beaten path in the country, the man who uses h'is eyes need never weary, or feel tedious, even when alone. And if o_ne be con_fined to the same spot, the changes in time _have JUSt as much variety and continual novelty m them, as the changes with the change of place. The spot must be a little one in which something ne~ shall n_?t. be met with ~very day: and whatever 1s found, If It be examined in its relations to other things, and to its own state previously, there will be knowledge obtained. 'l'he great difficulty lies in beginnincr. Few people have their att_entiot:I called to natural appearances and productwns, m that early period of life, when the ~nly object is the acquiring of knowledge purely for Its own sake. The natural desire which parents and others, who. hc:ve th_e care . of young people, have that the prehmmary mstructwn which 1s to prepare them for business should be uninterrupted and occupy_ their whole attention, naturally renders those parties rather averse to the observation of nature, as falling more within the category of play than of that of business. Also, when the young do take a turn for that species of occupation t~ey are .apt to become inquisitive, and to put ques~ tlons which are not very easily answered, even by those w~10 ~now a little of the quality of natural his. tory whic_h 1s current m the printed books. Indeed, as th~ sc~e.nce of plants consists very much in the techmcC;thtles of a system, of which beginners cannot easllv see the use. either in acquiring a know. |