:i4 



ZONES AND REGIONS [Pt. Ill, Sect. ] 



season in tropical continental districts, and must be of considerable im 

 portance in relation to geographical botany ; it is well known that ii 

 Bengal thin sheets of water freeze during the night in the dry season 

 In districts on the coast, in the forest, and on the hills the cooling inducec 

 at night-time by radiation is much less, thanks to the large amount o 

 water-vapour in the air, although physiologically it is by no means in 

 significant. 



The following table gives a summary of mean temperatures in paralle 

 belts of the tropical zone : — 



MEAN TEMPERATURE IN PARALLEL BELTS OF THE TROPICAL 



ZONE' (after Spitaler). 



Latitude 



Xorth. 



25° 



15" 



South. 



15" 



25° 



January 



1 8-4 



217 



23-9 



257 26-2 26-2 



26-1 



25.9 



257 



25-2 



24.7 



July 



28-0 



28-1 



27-9 



267 26-1 25-5 



24-9 



24-0 



22-6 



20-5 



I8-I 



Year 



237 



257 



26-3 



26-4 26-1 25-9 



25-5 



25-0 



24-2 



227 



209 



iii. LIGHT AND THE ULTRA-VIOLET RAYS. 



The intensity of the rays of light, like that of the heat-rays, is naturall 

 greater within than beyond the tropics, and the tropical daylight 

 brighter than that of the temperate or polar zones. This feature ma 

 be directly observed in the clearer reflection from the surface of wati 

 and also from that of foliage when wet with rain, and it is \&xy noticeab 

 in photographs taken in the tropics. True as this is of the luminous ray 

 it is equally true of the chemically effective ultra-violet part of tl 

 spectrum. 



In the tropical districts of America and of the Asiatic islands that I know, t 

 intensity of light is however not so strong as in East Africa, regarding the conditio 

 of light of which P. Reichard writes as follows : ' In the natural features around i 

 the dazzling brightness of the air is most striking. The more vertical tropical si 

 produces a much brighter light than in Europe. At first the eyes can barely endu 

 the glare, so that one is obliged to wear blue spectacles and to draw one's 1 

 down over one's eyes-.' 



There are no direct comparative observations regarding the intensi 

 of luminous radiation for different latitudes, but some observations ha 

 been made regarding the chemical rays. Thus simultaneous observ 

 tions at Kew and at Para in Brazil on three' April days of the year iH. 

 showed an intensity of chemical action nearly twenty times greater at Pa 

 than at Kew ; even in August it was 3-3 times greater at Para. 



^ Hann, op. cit., Bd. II, p. 17. 



- Deutsche Rundschau, Oct. 1894. Quoted by Hann, op. cit., I!d. II, p. 40. 



