﻿338 Mr. J. K. Laughton on Barometric Differences 



temperature of the air diminishes as we ascend at the nearly uni- 

 form rate of 1° in 300 feet, whilst the noontide temperature di- 

 minishes much more rapidly. I have already shown that differ- 

 ences of temperature, such as we ordinarily find near the surface 

 of the earth, shading into each other through almost infinitesi- 

 mal gradations through very great distances, are insufficient to 

 produce any appreciable effect ; but I am none the less ready to 

 admit that the sudden expansion, equivalent to a rise in tempe- 

 rature of 60° F., of a column of air 5000 feet high and covering 

 a base of hundreds of thousands of square miles, partaking, as 

 it must do, almost of the nature of explosion, is capable of pro- 

 ducing an effect the vastness of which may be conceived without 

 entering into any detailed and not very exact calculations. 



Whether, therefore, we consider merely the withdrawal of the 

 elastic force, or the disturbance caused by this expansion, or, as 

 in fact we must do, both combined, it would seem conclusively 

 established that tropical rain is a cause sufficient to produce a 

 low atmospheric pressure in any given locality during the wet 

 season : but we are met by the very startling contradiction 

 offered by observations in the valley of the Amazon, where the 

 volume of water that rolls down the river swelled to 50 feet 

 above its level, whilst the whole country for many miles on each 

 side of the principal tributaries is inundated, tells of the extra- 

 ordinary precipitation, to which a total want of meteorological 

 registers prevents our assigning any numerical estimate. Mr. 

 Buchan, indeed, runs his mean isobar of 29*9 up the whole length 

 of the valley, across the Andes, and down to the Pacific on the 

 western side ; but admitting this as the mean height of the 

 barometer at Para, we cannot allow that it is also that of the in- 

 terior, unless we reject the observations of Chandless and several 

 other careful travellers. Wallace, in attempting to form an es- 

 timate of the height of various stations on the Rio Negro, says 

 that " the mean of five observations at Barra, some with river-, 

 some with rain-water, gave 212 0, 5 as the temperature of boiling 

 water," which is equivalent to a barometric height of 30*2 inches. 

 Chandless, again, says that on the Purus, " at Canotama, three 

 feet above the high-water line, the barometer (mean of sixteen 

 days at 9 a.m. and 3 p.m.) stood higher than in Manaos at 

 about 10 feet above the high-level of the River Negro at the 

 homologous hours." 



Mr. Chandless's observations were taken in July, Mr. Wal- 

 lace's in August, and compel us, in the absence of others with 

 superior claims to exactness, to believe that the barometer does 

 stand very high at that season of the year. But that season is 

 not the rainy season ; the rains on the Purus end in the early 

 part of June, when the whole country is under water, forming 



