﻿and Fluctuations. 331 



Another error due to the confusion between the ideas of den- 

 sity and elastic force is the opinion, widely put forward, that the 

 presence of aqueous vapour in air reduces the barometric pres- 

 sure, and that this becomes lower accordingly as the quantity of 

 vapour becomes greater. It is said that aqueous vapour is lighter 

 than air, and that, since a volume of air saturated with vapour 

 has less weight than an equal volume of relatively dry air sub- 

 ject to the same pressure, the barometric pressure of the moist 

 air is therefore necessarily less than that of the dry — as if be- 

 tween volumes of air balancing the same pressure there could be 

 any difference at all ! In fact the very important clause, subject 

 to the same pressure, is overlooked ; so that it is carelessly but 

 very commonly maintained that the mere presence of vapour in 

 the air accounts for a low barometer, or even that the introduc- 

 tion of vapour into the air causes a fall. How utterly opposed 

 this last idea is to fact can at once be shown by putting an ane- 

 roid under a glass receiver containing comparatively dry air, and 

 introducing water : the vapour thrown off raises the barometric 

 reading by the amount due to the vapour at the existing tempe- 

 rature, gradually increasing it till the air inside the receiver 

 becomes saturated. If in a wider field we refer the question to 

 geography, and inquire whether those regions of the earth where 

 the air is permanently saturated with moisture are the regions 

 where the barometer is also permanently low, we obtain answers 

 of the most widely differing purport. Great stress is usually laid 

 on the low barometer in the Doldrums of the Atlantic, or in India 

 during the season of the south-west monsoon ; but, on the other 

 hand, in the valley of the Amazon and some of its principal tri- 

 butaries, where, according to Mr. Bates, "one lives as in a per- 

 manent vapour-bath," we find travellers repeatedly noticing that 

 the barometric observations give a level scarcely above that of 

 Para*. Comparing these opposite instances with those already 

 mentioned, of air almost entirely free from moisture showing a 

 very high pressure, as in Siberia, or a very low pressure, as in 

 the Antarctic, we can scarcely form any other conclusion than 

 that neither the permanent presence of moisture in the air nor its 

 permanent absence produces any effect on the barometric mean. 

 This conclusion corresponds with that at which we have pre- 

 viously arrived with regard to long-continued high or low tem- 

 perature ; and though both rest entirely on geographical obser- 

 vation, it will be not uninteresting to examine into the cause of 

 results so contrary to the general, or rather to the popular belief. 

 In the reasoning which has been adopted, there is clearly a 



* Amongst many others, see Wallace, ' Travels on the Amazon and Rio 

 Negro,' p. 41.1 ; Chandless, in Journ. of the Roy. Geog. Soc. vol. xxxvi. 

 p. 94. 



