been supplied by artificial woods, which now render the aspect of the country very different from what it was in the days of Dr Johnson, of whose remarks on this subject we have heard so much.

In examining the places where our natural trees grow most abundantly and luxuriantly, one might be surprised to find that the soil is generally sterile and shallow. This, in fact, is almost universally the case. Excepting a few species of willow, the alder, and perhaps the bird-cherry, I do not know a single native tree that does not seem, by preference, to grow in sterile soil. We know, indeed, that the oak, the elm and the ash, attain a greater size, when planted in deep soil; and it might be said that the circumstance of our indigenous woods being generally upon barren ground, is no evidence of the predilection of trees for such ground, as they are merely the more stunted remnants of our ancient woods, which have escaped on account of their inaccessible situation. Yet the circumstance is an important one, and ought at least to prevent the planter from being discouraged by the sterility of a district. It is astonishing to see in numberless places, large ashes, rowantrees and poplars springing up from the crevices of roeks, or from among gravel and fragments of stones, where they do not seem to have a particle of such soil as is generally considered essential to vegetable luxuriance. All our native firs grow in shallow and gravelly soil. Even the roots of primeval firs, so common in many of our peat-mosses and moors, generally occur in the gravelly or clayey subsoils; and in those parts of the country where portions of the original woods yet remain, we never find that the trees have sprung from the peat which now forms the surface soil, but from the subjacent strata of primeval formation. If this fact be correctly stated, it follows, that, with reference to certain species at least, such as the fir, the birch, the aspen and the mountain-ash, it would be well in planting, to immerse their roots in the original subsoil, and where the superincumbent peat is not too thick, to clear it away for that purpose.

It is difficult to account for the fact, that trees do not now propagate naturally in Scotland, in places which were formerly covered by them. The fact itself is certain. In the Hebrides, where several species still grow in the crevices of rocks, the

birch, for example, the hazel-leaved elm, the aspen and the rowan, these trees never propagate from seed, and merely send out a few shoots from the root. In many of the inland glens, where we meet with decayed forests of alder and birch, a few years will leave the heathy and fern-clad peat-soil utterly destitute of trees. Peat-soil is obviously unfavourable to the growth of trees; and it appears probable that its accumulation in these places is the principal cause which prevents the native seeds from germinating, or the young trees from rooting.

There is no evidence of any decided change of climate, and any influencing cause arising from such a change is merely conjectural. But from what we know of the past and present state of this and other countries, we may fairly attribute the chief cause of the defective propagation or disappearance of native wood to a change in the nature of the surface soil.

Thus the central parts of Ross-shire were covered with pine. This is proved, not merely by tradition, but by ocular evidence, the roots still remaining in the ground. These roots are all found, in their natural position, in gravel, or clay, or the crevices of rocks. This original soil is now covered to a varying depth with peat, which we know increases rapidly in favourable situations, and which always increases when covered by the vegetables which naturally grow upon it in this country. If it is daily increasing, it must have had a commencement. Fir seeds sprinkled on deep peat will not grow, or will not give rise to permanent trees. Therefore, when the seeds from which sprung the trees, whose decayed roots are now found in the subsoil, were naturally sprinkled and germinated, what is now subsoil was surface-soil, or was but very thinly covered by peat. Again, fir-trees grow perfectly well at the present day, when planted in gravelly or sandy soil, or in such soil slightly covered by peat. Therefore, the soil fitted for that tree, as shown by this circumstance, and the situation of old roots, is gravelly soil.

The same reasoning applies to many other trees, in so far as regards peat, although the soil in which they thrive best may be different. And thus it would appear that the chief cause of the extinction of our natural woods in the Highlands, is the accumulation of peat.

But another question presents itself, with respect to the islands

and maritime districts:-How does it happen that there, when trees have been planted, and have vegetated vigorously for some time, they are yet stopped short the moment they get beyond the level of the sheltering wall or rock? Trees, it will be said, formerly grew in such situations. Thus there is a large firroot at the head of Loch Resort in Harris, washed by the high tides. But there is no evidence that, in such exposed situations along the shores, trees ever grew. With respect to the above root, it may be said that there are numerous submarine forests along the coast, for example, in Pabbay and Islay, where the roots retain their original erect position, although no one could imagine that they had ever grown in the sea. The inference therefore is, not that large trees formerly grew upon the exposed sea-shore; but that the roots now found there were those of trees formerly growing in inland situations, but which, by some geological revolution, became maritime by the submersion of the intermediate ground. The sea air is thus, after all, probably the cause which prevents the growth of trees on exposed parts of the coast. Trees planted in such places will grow, however, when sheltered by a screen of any kind, although they never thrive as in more inland parts. The violence of the westerly gales is also very injurious to them; and from both causes operating together, the hedges and shrubs about Ballintrae, on the Ayrshire coast, are miserably stunted, and all incline to the eastward. The predominating influence of the westerly winds is indeed felt in all parts of Scotland; and even in the neighbourhood of Edinburgh, all the tress that grow in open places have the same inclination.

The clearing of land for agricultural and particular purposes, is rendered necessary by the progress of civilization, which is fast extending her empire over the land. The great forests of all the lower districts have long ago vanished, and have been partly replaced by trees imported from the Continents of Europe and America. Even many of the Highland glens have been filled with these trees, and the few patches of natural wood left in the Lowlands have been so intermixed with planted trees, that one cannot separate the natural from the artificial in them. The time is perhaps not far distant when the remote districts will cease to produce an unmixed vegetation,

and when the botanist, wandering among their deep glens and by their secluded lakes, will see on the craggy cliffs the native products of the soil mingled with those of distant lands. But surely, no trees can be better adapted to the climate of Scotland than her own native trees; and it remains to be proved that the oak, the ash, the elm, the pine, the birch, and the mountain-ash, are not as valuable or as ornamental as many other trees which are made to supplant them.

ON WHEEL-CARRIAGES. BY WILLIAM GALBRAITH, Esq. M. AEdinburgh.

THE application of wheel-carriages to general purposes, and particularly to those of agriculture, forms an important object of discussion in every country where the useful arts are known and cultivated. The difficulty of moving or transporting heavy bodies on rugged and uneven surfaces, would early suggest the idea of sledges, which in certain circumstances are still advantageously employed. The friction attending these, however, would soon be felt, and would lead those frequently employing such machines, from the properties of sliding to those of rolling bodies, which are still less affected by friction. The rollers or wheels were originally attached to an axle, so as to revolve along with it, as at this day exemplified in the Irish cars. But a little experience and reflection would show that the friction would be less if the wheels were moveable about an axle, which itself remained fixed to the body of the carriage; and that this, besides diminishing friction, would likewise diminish the force requisite to move the carriage, by saving the power necessary to turn the axle which was formerly moveable, but would now be at rest.

A slight consideration of the subject will prove, that wheels, moveable about a fixed axle, have other advantages than this, such as those which arise from facility of turning in a narrow space, where one wheel is capable of moving forward, while the other may revolve backward. This evidently gives to wheels moveable about an axle, a decided advantage over those attached to it, as has been sanctioned by long experience. A

further advantage arising from this mode of fixing the wheels is, that the construction of the parts by which the axle is attached to the cart, is lighter, and less complicated, than by the other method.

Carriages have, in general, hitherto been moved by animals, and even sometimes by wind, but there is now a prospect of that powerful agent steam being applied to this purpose.

In discussing the subject of wheel-carriages, which is allowed to be attended with considerable difficulty, it is proposed to treat,

I. Of the proper formation of the several parts of which the carriage is constructed.

II. Of the agents generally employed as a moving power. III. Of the most advantageous methods of applying that power.

I. Of the proper formation of the several parts of which the carriage is constructed.

1. Wheel-carriages are generally divided into two kinds or classes—one having two wheels, and the other four, and both these have their peculiar advantages. For lighter purposes, those having two wheels are generally employed. They are more readily managed, quicker in their movements, and relatively perform a greater quantity of work with the same number of horses than the larger machines; while for the transportation of the more bulky and ponderous objects, those having four wheels are most convenient, and often absolutely necessary. The bodies of the carriages which are intended to contain the load supported upon the wheels, are, or ought to be, formed according to the species of loads they are likely to carry. When the loads are specifically light, the body should be large and full, so as to bring the centre of gravity as low as possible, to prevent the cart from overturning on an inclined road, when highly loaded with comparatively light materials, as hay, straw, and similar substances. For loads of great specific gravity in particular, that is, loads of considerable weight within a small bulk, the body of the carriage should be strong and compact, and every advantage taken of the materials of which