April 14, 1856.] HOPKINS. 59 



to any one of the above-mentioned " areas of condensation," which 

 happen all to be in the tropics, it acquires, from that cause alone, a greater 

 capacity for dissolving vapour, and becomes a drier wind. If it passes 

 over the sea, it supplies itself with moisture in its passage; but if over 

 a continent which yields little evaporation, the farther it travels the drier 

 does it become. 



The mountainous and rainy promontories of South America, South 

 Africa, and Van Diemen's Land, have a special influence upon the 

 dryness of the winds which blow over them to the northwards, for they 

 abstract a large portion of the vapour which they would otherwise have 

 carried along with them. 



From this reasoning Mr. Hopkins would conclude that every country 

 which lies between the above-mentioned " areas of condensation," and 

 any great extent of continent stretching towards colder latitudes, is ne- 

 cessarily traversed by a dry wind, and thereby becomes arid and desert. 

 It is thus that he accounts for the deserts in either hemisphere. 



He concludes by hoping that future travellers will not omit to observe 

 the force, direction, and moisture of the wind in the various countries 

 they visit. Observations made at sunrise, and again at the hottest time 

 of the day, say between one and two o'clock, would be of value. 



Mr. Hopkins' paper is of considerable length, and enters minutely 

 into details respecting the physical features that influence the aridity of 

 each desert tract. 



Captain FitzRoy said that the subject of the paper was too difficult to 

 be dealt with in a short space of time. He could wish that the paper had 

 been divided into distinct heads, so that each might be more easily referred to. 

 With the main argument of the paper he should take issue at the very 

 outset. That argument was based on the assertion that " condensation of 

 vapour causes a vacuum.'''' Now, some of the ablest men who had studied 

 the effect of the presence of aqueous vapour upon the weight of the atmo- 

 sphere, had come to entirely different conclusions respecting it. Whether 

 therefore aqueous vapour adds to or diminishes the weight of the atmosphere, 

 is a disputed point. Some add a correction to the weight of a column of 

 air, on account of the vapour included within it ; while others (and a very 

 large number) contend that this very correction ought to be suhti^acted from 

 the weight of the given column. That no vacuum is produced by the con- 

 densation of aqueous vapour in the atmosphere is shown by the barometer, 

 which does not sink after a fall of rain, but, on the contrary, rises, showing 

 that the atmosphere has become heavier. Where moving masses of air meet 

 with high lands, there is no doubt that their moisture is condensed : therefore 

 it is that the windward side of mountains is usually clothed with forest, and 

 abounds with water, its fertility being consequent upon its rainy climate, while 

 the opposite side is characterized by dryness and by a clear sky. Among 

 many examples that might be given, he would instance the Galapagos Islands 

 (a group in which the evidence of volcanic agency was very abundant) ; here 

 the wind is perennial, from the eastward ; consequently, the windward sides 

 of the mountains on these islands are covered with forests to their summits, 

 while, on the opposite sides, the lava is still as sharply edged as it ever was, 

 cutting the shoes of those who walk over it. The whole subject of atmospheric 



