We may advert, as an illustration, to the feeding properties of the turnip.  It is usual to reckon the value of a crop of turnips by the number of tons per acre which it is found to yield when so many square yards of the produce are weighed.  But this may be very fallacious in many ways.  If they are white turnips, for instance, nine tons of small will contain as much nourishment as ten tons of large—­or twenty-seven tons an acre of small turnips will feed as many sheep as thirty tons per acre of large turnips.  Or if the crop be Swedes, the reverse will be the case, twenty-seven tons of large will feed as much stock as thirty tons of small.—­(Vol. ii., p. 20.) Mr. Stephens points out other fallacies also, to which we cannot advert.  One, however, he has passed over, of equal, we believe of greater, consequence than any other—­we allude to the variable quantity of water which the turnip grown on different soils in different seasons is found to contain.

It is obvious, that in so far as the roots of the turnip, the carrot, and the potatoe, consist of water, they can serve the purposes of drink only—­they cannot feed the animals to which they are given.  Now, the quantity of water in the turnip is so great, that 100 tons sometimes contain only nine tons of dry feeding matter—­more than nine-tenths of their weight consisting of water.  But again, their constitution is so variable, that 100 tons sometimes contain more than twenty tons of dry food—­or less than four-fifths of their weight of water.  It is possible, therefore, that one acre of turnips, on which only twenty tons are growing, may feed as many sheep as another on which forty tons are produced.  What, therefore, can be more uncertain than the feeding value of an acre of turnips as estimated by the weight?  How much in the dark are buyers and sellers of this root?  What wonder is there, that different writers should estimate so very differently the weight of turnips which ought to be given for the purpose of sustaining the condition, or of increasing the weight, of the several varieties of stock?  Other roots exhibit similar differences; and even the potatoe, while it sometimes contains thirty tons of food in every hundred of raw roots, at others, contains no more than twenty—­the same weight, namely, which exists at times in the turnip. [4]

[Footnote 4:  For our authority on this subject, we refer to Johnston’s Suggestion for Experiments in Practical Agriculture, No. 111. pp. 62 and 64, of which we have been favoured with an early copy by the author.]

This latter fact, shows the very slippery ground on which the assertion rests, that has lately astonished the weak minds of our Southern cattle-feeding brethren, from the mouth of one of their talented but hasty lecturers—­that the potatoe contains two or three times the weight of nourishment which exists in the turnip.  It is true that some varieties of potatoes contain three times as much as some