Nature and Cause of the Potato Disease. 
307 
governed by similar circumstances, and, therefore, alike in action. 
A fine day with sun and wind always promoted evaporation, while 
a dull cold day retarded or entirely stopped it. The leaves of 
the polyanthus at the end of the 12 days were again measured, 
when the extent of one surface was found to be 67'26 square 
inches, that of the first measurement being 66 54, an increase too 
trifling to affect the calculations. 
On the 8th and 9th of March, which were bright sunny days, 
the thermometer on the 8th at 6 a.m. stood at 36°, at 9 a.m. 50°, 
and at noon 83°, and on the 9th at 7 A.M. 32°, and at 9 a.m. 46°. 
In those two days the mould alone evaporated 250 grains, and 
the polyanthus 1500 grains, which is 5'7 grains for each square 
inch of the mould, and 4'83 for the polyanthus, including both 
surfaces of the leaves. On the r2th of March no evaporation 
took place during the day and following night ; the weather at 
this period was dull and cloudy, attended with absence of sun, 
some rain, but no wind. The thermometer at 8 a.m. on the 12th 
stood at 42°, at 5 p.m. 46°, and on the morning of the 13th at 7 
A.M. it was 36°. These experiments were continued until the 
14th of April, a period of 46 days, when the results for the whole 
of that time were, that the polyanthus evaporated 17'500 grains, 
and the mould ll'OOO grains, leaving 6"500 grains to be ac- 
counted for as exhalation from the plant. The average daily 
evaporation therefore for every square inch of the mould is 10 8 
grains, and for each square inch of one surface of the leaves 2 01 
grains, and for both surfaces half that or 1 '005. 
By a similar contrivance to that used with the polyanthus, I 
experimented upon 3 potato plants ; the surface of one side of 
the leaves measured 46 square inches, while that of the mould 
equalled 132 inches. I commenced the experiment on the 19th 
of March, and concluded it on the 14th of April following, a 
period of 19 days. 
The whole amount of water exhaled was 57"000 grains. This 
includes the leaves and stems of the plants and the surface of the 
mould as well. The evaporating power of the mould during that 
period equalled 10'8 grains for every square inch daily ; this for 
132 inches during 19 days, gives 54'000 grains for the mould, 
and 3000 for the plants, which, for each square inch of one surface 
of the leaves, is 14 grain daily, and for both surfaces /j, of a 
grain only. The exhalation from the potato was therefore much 
less than from the polyanthus during the same period. Thus 
the potato gave 1-4 grain and the polyanthus 2*01 grains for 
every square inch of one surface daily. During the whole period 
of these experiments no rain or water, except that which has 
been noticed, came in contact with the plants and vessels ; but 
although protected from rain, a free circulation of air was per- 
