22 REPORT — 1844. 



from alterations of surface temperature, but from the condensation of aqueous vapour, 

 and the consequent formation of rain. This, he said, caused local heatings of the 

 atmosphere and considerable reductions of its pressure in the locality, particularly in 

 the colder latitudes. Within the tropics, the barometer does not ordinarily fall as 

 much as in colder latitudes, notwithstanding the abundant rains which take place 

 there, because the condensation occurs, and the temperature is increased at a greater 

 height in the atmosphere, and the reduction of the incumbent pressure in the part is 

 spread over a wider area. The condensation takes place too at an elevation, where 

 the air, from being subjected to inferior pressure, is more attenuated, and the heating 

 is consequently more diffused. Rain is formed in certain latitudes, say at an average 

 height of 3000 feet, where the air has a density proportioned to that height, and 

 where the whole effects of the local heating are confined to an area of moderate ex- 

 tent, thus reducing the pressure of the atmosphere on the barometer in every part of 

 that area in a considerable degree ; whilst, in other parts nearer the equator, the 

 condensation which produces rain takes place at an average height of, say 6000 or 

 9000 feet, where the air is rare in proportion to the height ; the heating 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 climates. 



On the Diurnal Variations of the Barometer. By T. Hopkins. 



Mr. Hopkins 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 constitute 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 locahty to begin to fall at, say about ten o'clock in the morning, 

 which fall continued until about four o'clock in the afternoon, 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 some- 

 what 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 fallings in the twenty-four 

 hours. This was shown to be in general accordance 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 continued 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 sta- 

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

 declining temperature until the great cold resulting from evaporation ceased. The 

 diurnal fluctuations were also shown to he the least, when the irregular were the 

 greatest (as observed by Mr. Birt), because rain was then produced, and evaporation 

 prevented from cooling the air at the regular diurnal period, and in that way rain pre- 

 vented the rise of the barometer at that recurring period. 



A Years Meteorological Observations made at Aden. By Sergeant Mayer. 



On tlie Temperature of the Air at various Soundings of Huggate Well, upon 

 the Wolds of the East Riding, Yorkshire. By the Rev. T. Rankin. 

 This well is 116 yards, or 348 feet deep. On Saturday, September 21, 1844, at 



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