Proceedings of the British Association. 433 



to that height, and where the whole effects of the local heating are con- 

 fined to an area of moderate extent, thus reducing the pressure of the 

 atmosphere on the barometer in every part of that area in a consider- 

 able degree ; whilst, in other parts nearer the equator, the condensa- 

 tion which produces rain takes place at an average height of, say 6,000 

 or 9,000 feet, where the air is rare in proportion to the height ; the heat- 

 ing effects are, therefore, diffused to a corresponding extent, whilst the 

 reduction of pressure at the surface is spread over a wider area. It 

 follows, that with equal amounts of rain, the fall of the barometer will 

 be the greatest, and confined to the smallest area, in the coldest cli- 

 mates. Mr. Hopkins also represented that the diurnal oscillations of the 

 barometer arise from, first, the condensation of aqueous vapour into 

 cloud, and then from the evaporation of the particles of water that con- 

 stitute that cloud. He stated, that the morning sun warmed the lower 

 air, and caused it to rise until condensation formed cloud, and liberated 

 heat sufficient to warm a mass of the atmosphere, and thus to cause the 

 barometer in the locality to begin to fall at, say about ten o'clock in 

 the morning, which fall continued until about four o'clock in the after- 

 noon, when condensation ceased. From this time, evaporation of the 

 cloud commenced which cooled the air in the part — made it heavier — and 

 caused the barometer to rise until about ten o'clock, p. m., by which 

 time the cloud was evaporated. The cooled and heavier air now de- 

 scended to the surface, from which it absorbed a portion of heat, and 

 became somewhat warmer. From this second warming of the air, and 

 from a reduction of the quantity of aqueous vapour in the atmosphere, 

 as is evidenced by the fall of the dew point, the barometer again fell, 

 and from the operation of these two causes, continued to fall until four 

 in the morning ; from which time, those general cooling influences that 

 operate in the absence of the sun, caused the barometer again to rise 

 till ten in the morning, thus completing the two risings and two fall- 

 ings in the twenty-four hours. This was shown to be in general ac- 

 cordance with the tables of the Plymouth observations for three years, 

 and with those made at Madras and Poona. The fact, also found in 

 the Plymouth observations, that the dew-point rose with the tempera- 

 ture until eleven o'clock, a. m., when, although the temperature conti- 

 nued rising, the dew-point did not rise higher, showed that the vapour 

 formed during the hottest part of the day was expended in supplying 

 that which was condensed in forming the daily cloud. According to 

 these tables, also, the dew point at the surface continued stationary 

 until four o'clock, p. m. when it began to fall, and continued falling 

 with the declining temperature until the great cold resulting from eva- 



