The fattening of sheep on turnips is much promoted by their having access to a grass field, more especially if it happen to contain whins or heather. It is from want of attention to this that sheep are so liable to disease when eating turnips, for, apart from the benefit that accrues to them from a dry lair, they are enabled to turn their food to better account when consuming bitter herbs. It is no unusual thing for turnip-fed sheep and cattle to become quite lean, as the farmers say, "almost at the lifting," for no other reason than that they have been confined too strictly to one article of diet. They have been denied access to plants containing of all things the one most necessary for the maintenance of their health--_bitter extractive matter_--as it is called by chemists--without a due proportion of which the most nutritious substances cannot be turned to account. "As an essential ingredient in the provender of herbivorous animals, it may, I think, be admitted as a fact, that its importance is _in an inverse ratio_ with the nutritive powers of the food."[22] Thus accounting for the length of time that sheep will continue to thrive on turnips alone. [22] Paris's Pharmacologia, sixth edition, vol. i. p. 147. With all the advantages, however, which accrue to the sheep when on turnips, from the quantity of nutritive matter which these roots contain, its progress when restricted to them frequently falls very far short of the expectations of the owner. In the greater number of instances, also, farmers are unable to account for their want of success in this department, so that I may be excused for endeavouring to point out, at some length, the causes of their failure. To proceed:--