Brea 
1878.] The Beneficial Influence of Plants. > 805 
from his results will be sufficient for our purpose, namely: “ When 
the tension of the vapor was 0.888 inches the fall of the ther- 
mometer was 6.0° Fahr., and when the tension was only 0.435 
inches the fall amounted to 16.5° Fahr.” It is fair to state that 
these observations were. conducted long before Tyndall's 
researches with aqueous vapor, and are on this account all the 
more valuable. The evidence we have adduced seems to show 
conclusively, that the vapor forms a sort of invisible canopy sepa- 
rating the chilling air above from the warm earth beneath, and 
arresting, more or less effectually, the radiation. We have a 
homely illustration of this when we see the thrifty housewife 
spread coverings over the more delicate plants of her flower gar- 
den on nights when she fears a frost. It has been said that 
aqueous vapor is a blanket more necessary to the vegetable life 
of England than clothing is to man; and every plant capable of 
destruction by freezing would sageu mb, if this vapor were 
removed for a single night. In support of this assertion it will 
be well to notice the results obtained by some noted observers 
concerning the daily fluctuation in temperature in other countries. 
Dr. Livingston? has observed a great excess in nocturnal chill- 
ing when the air is dry over that which occurred when it is laden 
with moisture. He has found in the southern central portion of 
Africa, during the month of June, the thermometer early in the 
mornings at from 42° to 52° Fahr., at noon 94° to 96° Fahr., or 
a mean difference of forty-eight degrees between sunrise and 
mid-day. He says, furthermore: “The sensation of cold after 
the heat of the day was very keen. The Balonda at this season 
never leave their fires till nine or ten in the morning. As 
cold was so great here, it was probably frosty at Linyanti; I 
therefore feared to expose my young trees there.”? Crossing the 
continent, Dr. Livingston reaches the Zambezi at the commence- 
‘ment of the year. He gives the following description of the 
change felt on entering the valley of the river: 
We were struck by the fact that as soon as we came between 
the range of the hills which flank the Zambezi the rains felt 
warm. At sunrise the thermometer stood at from 82° to 86° 
Fahr.; at mid-day in the coolest shade, namely, in my little tent, 
under a shady tree, at 96° to 98° Fahr., and at sunset at 86° 
Fahr. This is different from anything we experienced in the 
interior. ae 
1 Livingston’s Travels, p. 484. 2 Livingston’s Travels, p. 484.5 
