142 
On Modes of Comparing the 
go. Column No. 5 is founded on Von Thaer's table, in which 
he takes 100 lbs. of ordinary hay as having given a certain amount 
of nourishment ; and then finding, from similar experiments, 
200 lbs. of potatoes, and 397 of beet, to give an equal amount, he 
calls them equivalents. The value of the turnip (viz. 500) is from 
an extended table of Professor Johnston's, based on exactly the 
same data. Swedes unluckily are not given. 
Any immediate practical utility, from ascertaining the specific 
gravity to be a measure of nourishment, does not apply so directly 
to the above comparison between different roots as between varie- 
ties of the same root. The relative power of nourishment of dif- 
ferent roots is pretty well ascertained; and if it is not, it is a 
matter of such obvious and general importance that more certain 
modes of learning it will of course be resorted to. Not so with 
the vast varieties of the same root, nor with the same sort in dif- 
ferent soils, for which a quick and easy mode of taking their rela- 
tive value would be desirable ; for it will be rare that any one 
would be at the expense of having a number of chemical analyses 
merely to ascertain the different degrees of nourishment of the 
rohan, or Welsh kidney potato ; nor certainly would any one be 
at the trouble of a series of experiments in feeding sheep on 
turnips, to put half-a-score on the Pomeranian globe, and another 
half-score on the border imperial ; and yet these experiments 
offer some very curious details on these points, which scarcely 
seem to be the result of accident. Thus it becomes a matter of 
more importance to ascertain the general position than might 
have been at first sight imagined. 
The following tables exhibit the different specific gravities in 
varieties of the same root : — 
Potatoes. 
Average 
Specific Gravity 
of C Tubers. 
1841. 
Average 
Specific Gravity 
of 12 Tubers. 
1812. 
Rohan .... 
Purples .... 
Welsh kidney 
1081 
1091 
1107 
1089-95 
1102 
1105 
Now it is the received opinion among those who cultivate the 
potato in this district that the Welsh kidney requires a much 
richer soil than the purple, which is the common sort grown for 
ccmsumption here — an opinion borne out by some experiments 
of mine, with which great pains were taken in the cultivation of 
