THE ENGLISH LAKE DISTRICT. 121 



Cirrostratus above the valley. When the mountain is enveloped in mist quite 

 down to its base, particularly if accompanied by deposition, the difference in 

 temperature does not exceed 7 or 8 degrees ; while, under a perfectly clear and 

 cloudless sky, it amounts to 18 or 20 degrees. By examining the Hygrometri- 

 cal Tables in connection with the simultaneous records of the state of the weather, 

 the variation of temperature under various conditions of the atmosphere as to 

 clearness and humidity may be computed. 



The effect of dense mist in assimilating the temperature of the different strata 

 of the atmosphere (by emission of its latent heat during condensation) is shown 

 in the observations of September 9th, 1847, taken on Snowdon in North Wales, 

 when the depression in 3571 feet was scarcely 10 degrees. 



Dr Buff gives the temperature on the Brocken, in 51 0, 51' north, 3500 feet 

 above the sea, 33 5, 8 ; at Gottingen, 51*30' N. 480 feet above the sea 47° — a dif- 

 ference of 13°2 in 3020 feet ; and at Munster, in the same latitude, 190 feet 

 above the sea, 49 D *1 — a fall of 15 *3 in 3310 feet. In tropical latitudes, the low- 

 ering of the temperature is much less sudden. On the table-lands of Mexico, 

 at 6990 feet, the mean temperature, according to Humboldt, is 61°-34 ; and at 

 Vera Cruz, on the sea-coast, in the same latitude, it is 77 degrees. At Quito, 

 nearly under the equator, (0 , 7 / S.) 8970 feet elevation, the temperature is 591, 

 and, at the coast, 78- 8 degrees. 



The conditions of the decrease of temperature in ascending on the earth's 

 surface and from the earth's surface seem to be very different. 



From the observations made during four balloon ascents at Kew in the 

 autumn of 1852, the number of feet of height corresponding to a fall of 1° 

 Fahrenheit was, as under: — 



On August 17th, . . . 292-0 Feet. 



20th, . . . 290-7 ... 



On October 21st, . . . 291-4 ... 



On November 10th, . . . 312-0 ... 



All the ascents, however, show an increasing degree of humidity up to the 

 altitudes of the highest English mountains. 



In a former paper, I ventured to remark, " The degree of humidity increases 

 upwards from the earth's surface, and the condition or combination of condi- 

 tions most favourable for the condensation and precipitation of vapour in the 

 greatest abundance will probably be found somewhere about 2000 feet above the 

 sea-level. It is assumed that the atmosphere is generally near the point of 

 saturation at and above 2000 feet ; but as the air temperature decreases with 

 every farther increase of elevation, its capacity for vapour is proportionately 

 diminished, and consequently there will be less to precipitate than at the point 

 where the temperature of the air and that of the dew-point first begin to balance 



VOL. XXI. PART I. 2 K 



