92 
Drought o/1870 and 
advantage of the results so obtained, it is proposed, in the 
present paper, to consider biiefly : — 
1. The probable amount of water exhaled during growth by 
some of our most important crops. 
2. The source whence the required supply of water is obtained. 
3. The difference of the effects of the drought of 1870 on 
the different experimental crops. 
Amount of Water (;iven off by Plants during Growth, 
A series of experiments was commenced in 1849, and was 
continued for ten years, to determine the amount of water given 
off by plants during their growth, in relation to the amount of 
the various constituents they assimilated. Of agricultural plants, 
wheat, barley, and mixed grasses, as representatives of the 
Graminaceous family ; beans, peas, and clover, of the Leguminous 
family ; and swedes, white turnips, mangolds, potatoes, and 
artichokes, as root-crops, were thus experimented upon. Similar 
experiments were also made on the exhalation by evergreen 
and deciduous trees, six of each being selected. 
The plan of experimenting was as follows : — Cylindrical 
vessels, first of glass and afterwards of zinc, 14 inches in depth, 
9 inches in diameter, and holding about 40 lbs. of soil, were 
employed. Soil from the plot in the experimental wheat-field 
which had grown 10 successive crops without manure was selected. 
The general rule was to make three experiments with each descrip- 
tion of plant ; one with the above soil without further addition ; 
one with the same soil with purely mineral manure added ; and the 
third with the same soil and both mineral manure and ammonia- 
salts in addition. In the cases of wheat and barley, plants from 
three seeds, and of beans, peas, and clover, one plant only, were 
planted in each vessel. A glass plate, having a hole in the centre 
about three-quarters of an inch in diameter for the plants to grow 
through, and another smaller one, closed at pleasure by a cork, for 
the supply of water, were then firml y cemented upon the top of each 
vessel. One vessel, supplied with soil and fitted with a glass 
cover like the rest, was, however, always left without a plant, in 
order to ascertain the probable amount of evaporation from the 
surface of the soil itself, through the centre orifice, independently 
of growth ; though, in the experiments with plants, the hole 
was always partially closed, by laying small pieces of glass 
over it as far as the stems would allow. Of course in experi- 
menting with root-crops the holes in the glass covers were larger, 
but they were kept closed around the plants as far as possible, 
in the manner just described. 
The vessel with its contents, weighing more than 40 lbs., was 
weighed from time to time, generally every ten days during 
