1878. ] The Beneficial Influence of Plants. 797 
or about two and one-fourth inches to be the yearly depth. Now 
if all the water transpired from this leaf surface were given off 
from a surface equal to that of the land area, we should find on a 
little reckoning that the depth of transpiration would be nine 
inches. It is to be remarked that transpiration from the grasses, 
cereals, underbrush, etc., was not considered in these calculations, 
but there can be no doubt as to their great assistance in this pro- 
cess; so that, were it possible to form anything like a correct 
estimate of the amount exhaled by these humble specimens of 
the vegetable kingdom the ratio would be greatly increased. 
In our Southern States, many of which have as high as fifty 
per cent. of woodland, the depth for plant exhalation must be 
much greater, equivalent to at least eighteen inches, since the 
conditions are so much more favorable. All will agree that this 
is no mean showing for transpiration as affecting the proportion 
of moisture in the atmosphere, the yearly average rainfall being 
forty-three inches at Philadelphia. 
There are evidently many inaccuracies in a computation like 
the foregoing, which it is exceedingly hard to avoid, but the 
allowances made will, it is hoped, meet the discrepancies and 
thus the true mark will not have been overreached. 
Provided our reasoning be at all correct, the objection of our 
philosophic critic cannot have any weight, and the important fact 
remains established, that sransfiration is capable of increasing the 
humidity of the air. Now, if transpiration so materially affects the 
quantity of the moisture in the outer air, what must be the effect 
of keeping plants in closed apartments? This question will 
here be discussed, and especially the effect of plants on the air of 
rooms heated by hot-air furnaces. According to the above 
extract it will be seen that the process is only about half as active 
indoors as in the open air, during the day, but at night the rate 
of transpiration is about equal in the two situations, so that 
during the whole twenty-four hours the quantity a plant would 
transpire indoors exceeds half what it would transpire in the open 
air, and we may presume from this fact alone that plants in rooms 
would influence the relative humidity of the air of the rooms. 
From observations which I have made over a period of several 
weeks on the air of my private reading and sleeping room at the 
Episcopal Hospital (Philadelphia), which is kept warm by air 
heated by steam, and simultaneously on the air outside, it was 
