SOG 
Nature and Cause of the Potato Disease. 
From this brief outline of the laws of evaporation, I shall pro- 
ceed to state the particulars of some experiments instituted for the 
purpose of ascertaining the amount of exhalation from the earth 
as compared with that thrown off by plants during the same 
period of time. 
I arranged an apparatus that contained two double metallic 
vessels of equal size, which were used as mould-pots. These vessels 
were so constructed that no moisture could escape, except at the 
surface ; while all superfluous water drained through to the 
reservoirs beneath, and could be returned again at pleasure. 
Each vessel had a gauge, that indicated the water in the reser- 
voir. One pot was filled with mould only, the other with mould 
containing a polyanthus, and, thus arranged, they were each placed 
separately on a peculiarly formed balance, and connected with 
graduated vessels containing water. Thus contrived, they were 
balanced and charged with a given quantity of water from the 
graduated vessels, the indices of which could be read off at plea- 
sure. They were now connected with the graduated vessels by a 
contrivance that made them self-acting. 
Each pot contained 22'09 square inches of surface at the level 
of the mould. 
The polyanthus contained 29 leaves, one side of which mea- 
sured 66 54 inches at the commencement of the experiment, 
consequently both surfaces equalled 133 08 inches. The machine 
was set to work on the 28th of February in the present year, and 
regularly observed once or twice a-day, and its action and the 
temperature of the atmosphere registered. In twelve days from 
its commencement the mould alone had evaporated 1600 grains, 
which is 6'06 grains for every square inch ot surface daily, while 
the polyanthus had evaporated 5250 grains, which, for the mould 
and one surface of the leaves, is 4 93 grains for every square inch. 
Deducting the evaporating power of the mould, 1600 grains, 
from that of the polyanthus and its mould, we have 3650 grains 
for the evaporating power of a plant containing 133 08 square 
inches in both surfaces of its leaves, which is equal to 2 28 grains 
for each square inch daily. Each day, however, was observed 
and the exhalation noted down, and, as might have been expected, 
the results varied. In cold sunless days the evaporation was 
cither much lessened or entirely checked ; while in warm sunny 
days it was the most active. I'he 1st and 2nd of March were 
dull cloudy days, and although the thermometer stood at 48° at 
7 A.M., no exhalation took place during either of the days. The 
3rd was a fine sunny day with wind, and the temperature 52°, 
or 4° higher than on the two preceding days. Here evaporation 
occurred to the extent of 198 grains from the polyanthus, and 
290 grains from the mould-vessel. The succeeding days were 
