Vol. 50.] GLACIAL GEOLOGY OF MOUNT KENYA. 527 



V. Climatic Conditions during the Period of Maximum 

 Glaciation. 



The former distribution of the alpine flora of Equatorial Africa 

 is an indication of the different climatic conditions that resulted 

 from or were concomitant with the maximum glaciation. An 

 attempt will be made in this part of the paper to determine the 

 meteorological conditions of Equatorial Africa at that period. 



In the first place, it will be advisable to consider the changes in 

 the general conditions of the atmospheric circulation, that would 

 have resulted from the elevation of Mount Kenya for over 5000 

 feet, and then the effects of these changes on the atmospheric 

 pressure and thus on the winds and rains. 



The first change to be noticed is that the whole of the uplifted 

 region would be colder. The average rate of decrease is generally 

 taken, following Herschel, at 1° E. for every 300 feet of ascent; 

 and though in many later cases which have been more accurately 

 observed the rate of diminution of temperature has been increased, 

 it may be advisable to take this rate — so as not to exaggerate the 

 amount. But it must be remembered that the rate of cooling 

 increases both with the ascent and the seasonal descent of the 

 isotherms. The annual mean, however, for the carefully collected 

 data given by Haun 1 from the observations on the Theodule, when 

 reduced to feet and Fahrenheit, also gives 1° F. for 300 feet. As 

 these results were obtained at the height of over 10,000 feet, and 

 well above the snow-line, the conditions of that case are probably 

 fairly analogous to those on the higher African peaks. 



The mean temperature will therefore have been 17° E. lower 

 than at present. This would have a treble influence : (1) the air 

 would contract in bulk ; (2) the saturation-point would be lowered, 

 and the air become drier ; (3) there would therefore, owing to the 

 increased precipitation, be more snow than under existing conditions. 

 Now all these three factors tend in the same direction, viz. an in- 

 crease of barometric pressure on the lower regions and a depression 

 of the isobaric surfaces. Consequently, there would be at night, when 

 the cold is greatest, an inset current at a high level toward the 

 mountain. This further helps the depression of the isobaric 

 surfaces, and there would thus be at night a downrush of air 

 along the slopes, similar to that well known in the Alps, with a 

 high-level inset current sweeping in to the mountain and carrying 

 the damp air from the surrounding lowlands on to the snowclad 

 summit. 



In the daytime the conditions would be reversed ; the sun would 

 exercise enormous power, the surface of the mountain would be 

 heated, ablation would be very rapid, the air would become moist, the 



1 J. Harm, ' Ueber das Kliraa der hochsten Alpenregionen,' Zeitschr. 

 oesterreich. Gesellsuh. Meteor, vol. v. (1870) p. 197. 



See also Harm's recent memoir, ' Weitere TTntersuchuugen iiber die tagliche 

 Oscillation des Barometers,' Denkschr. k. Akad. Wissensch. Wien, vol. lix. (1892) 

 pp. 297-356. 



