with Geographic Botany. — Humidity. 323 



supports wreaths of them which increase during its conti- 

 nuance. The aspects exposed to those refreshing winds, the 

 Trades, display a superiority in their vegetable productions, 

 arising from the moisture they are constantly conveying ; the 

 group of islands called the Galapagos, notwithstanding their 

 position, are not remarkable for fertility, but those situations 

 exposed to the trade-wind surpass the other parts of the 

 islands in the vigour of their vegetation. 



For some time it was an admitted circumstance, that the 

 quantity of aqueous vapour diminished in a regular progres- 

 sion from the earth upwards. But Mr. Daniell was led, first 

 of all, he tells us, by theory, and subsequently by direct ex- 

 periment, to consider that this was not the real state of its 

 suspension. The chief experiments on which he founds his 

 conclusions were made in an aeronautic voyage in the month 

 of September. On attaining an elevation of 9890 feet the 

 dew-point was exactly the same as at the surface of the earth; 

 and on ascending 1100 feet higher, the dew-point had fallen 

 thirty-two degrees. Some further observations were made at 

 elevations, the greatest little more than half the height of this, 

 and as none of them reached the point of sudden depression, 

 they are destitute of the chief part of their interest. Consi- 

 dering this as the correct view of its conditions, we have 

 nothing to add as to the circumstances connected with its 

 regular diffusion. 



II. The aqueous vapour of the atmosphere is continually 

 meeting with circumstances which disturb its suspension ; 

 depressions of temperature are of course the most usual, and 

 will be frequent in proportion as it approaches saturation. It 

 is evident that changes of temperature occurring near the 

 surface of the earth are soon propagated through the atmo- 

 sphere, as is shown in some of the causes influencing the for- 

 mation of dew. Howard was of opinion that rain was gene- 

 rally produced by electrical action, and many of the instances 

 which externally seem to produce rain by decreasing the 

 temperature, are capable of being attributed to alterations in 

 the electrical conditions. On the contrary, dews are deposited 

 solely by wanting a sufficient temperature to maintain their 

 suspension. The agents influencing this, and the circum- 

 stances attending the deposition, are among the most inter- 

 esting, and associated by all with the name of Dr. Wells. 

 It has been shown that the diminution of temperature has 

 not its origin in the air, since by terrestrial radiation the dif- 

 ferent substances on the surface of the earth become cooled 

 below the atmosphere, and as portions of it come in contact, 

 they part with that moisture which the diminished tempera- 



