The part of this chapter, again, which relates to the theoretical construction—­to the history of the successive improvements, and to the discussion of the relative merits of the numerous varieties of ploughs which have lately been recommended to notice—­is drawn up by Mr. James Slight, curator of the museum of the Highland Society, a gentleman whose authority on such subjects stands deservedly high.  To this monograph, as we may call it, upon the plough, we may again refer as another illustration of the union between agriculture and science.  Mechanism perfects the construction of instruments, chemistry explains the effects which they are the means of producing in the soil—­says also to the mechanic, if you could make them act in such and such a way, these effects would be more constantly and more fully brought about, and returns them to the workshop for further improvement.  Thus each branch of knowledge aids the other, and suggests to it means of still further benefitting practical agriculture.

One of the most interesting, and not the least important, of those practical discussions which have arisen since the establishment of the Royal Agricultural Society of England, has been in regard to the relative merits and lightness of draught of the Scottish swing-ploughs, and of certain of the wheel-ploughs made and extensively used, especially in the southern counties.  It is admitted, we believe, on all hands, that a less skilful workman will execute as presentable a piece of work with a wheel-plough, as a more skilful ploughman with a Scotch swing-plough.  This is insisted upon by one party as a great advantage, while the other attaches no weight to it at all, saying, that they find no difficulty in getting good ploughmen to work with the swing-plough, and therefore it would be no advantage to them to change.  Still this greater facility in using it is a true economical advantage, nevertheless; since that which is difficult to acquire will always be purchased at a dearer rate; and in an improving district, it is some gain, that it is neither necessary to import very skilful ploughmen, nor to wait till they are produced at home.

But it is also conceded, we believe, that the swing-plough, in skilful hands, is more easily or quickly managed than a wheel-plough; that it turns more readily, and when doing the same kind of work, will go over the ground quicker, and consequently do more work in a day.  Theoretically, this seems undeniable, though it does not appear to be as yet clearly established in what precise proportion this theoretical acceleration ought to increase the extent of ground gone over by a diligent ploughman in the ten hours of his daily labour.  It is said that, with the wheel-plough, three-fourths of an acre is an average day’s work, while with a swing-plough, an acre is the ordinary and easy work of an active man on soil of average tenacity.  The pace, however, must depend considerably both upon the horses and their driver; and to whatever extent such a difference may really exist—­and opinions differ upon the subject—­it is clearly an argument in favour of the swing-plough.