﻿336 Mr. J. K. Laugh ton on Barometric Differences 



thermometer or hygrometer : the calculations I have just made 

 show with equal certainty that the forces called into action by 

 the differences of expansion or of density due to mean«thermo- 

 metric or hygrometric conditions are quite inadequate to produce 

 any sensible motion; we are therefore fully warranted in the 

 conclusion that permanent high temperature, permanent humi- 

 dity, or the conditions most opposite to these, are not the causes 

 of local differences of barometric pressure. 



Part II. The Geographical Distribution of Barometric 

 Pressure. 



It is not, however, to be doubted that mean barometric pres- 

 sures have a very marked geographical distribution ; and though 

 concerning these our information is very incomplete, and, even 

 where it might seem complete, is frequently incorrect, we are 

 still able to determine some of the principal features of this 

 distribution with tolerable accuracy. The best-known instances 

 of it are perhaps the low barometer in the Doldrums of the 

 Atlantic, in India during the summer months, in the Antarctic 

 and in parts of the Arctic ; and, on the other hand, the high 

 barometer in the upper valley of the Amazon and its tributa- 

 ries, in a patch of the North Atlantic between the Azores and 

 Bermuda, in a corresponding patch of the South Atlantic to the 

 north and north-west of Tristan d'Acunha, and in Eastern 

 Siberia during the winter. There are other districts, perhaps 

 equally important ; but these I have named may be considered, 

 to a certain extent, representative areas, offering, at any rate, 

 very many of the most noticeable barometric phenomena. 



A consideration of the climatic characteristics of the Dol- 

 drums or of India seems to point to the heavy rains of these lo- 

 calities as affording some explanation of their low barometer. 

 Remembering the extent and absolute, as well as relative, 

 amount of the precipitation which, during the wet season, takes 

 place in India along the southern foot of the Himalayas, along 

 the whole of the west coast, and further east in the province of 

 Khasia, and thence southwards to the sea, it is difficult to over- 

 estimate the sudden disturbance to which the atmosphere is 

 subjected, whether merely by the removal of the vapour and the 

 consequent reduction of elastic force, or by the expansion follow- 

 ing the enormous development of heat by condensation. The 

 observations of this great rainfall are neither sufficiently nume- 

 rous nor consecutive to admit of our assigning to it any exact 

 measure ; but we are certainly far within the mark when we say 

 that it does not average less than one inch in the day during the 

 summer months. Now 1 cubic inch of water weighs 253 grs. ; 

 so that an inch of rain falling on an area of 1 square inch im- 



