with Geographic Botany. — Temperature. 



183 



to the west. It is due to the superiority of the south-west 

 aspect that Madeira is able to produce its Malmsey and Cer- 

 cial wines, for it is on this that the vine which yields them is 

 alone cultivated. 



To ascertain the value of this varying intensity, I took the 

 opportunity of the sun being very near the zenith to make 

 the observations contained in the table. 



Perhaps the results of these observations are not altogether 

 what might have been anticipated ; they will however show 

 incontestably the advantage of an inchned, or even of a hori- 

 zontal surface over a vertical one, m hen the altitude of the 

 sun is great. To convey an idea of the customary range of 

 the sun's altitude in the latitude of London, I add it on four 

 astronomical periods. 



Sun's meridian altitude. 



March 21, vernal equinox . . . 



June 21, summer solstice . . 



September 23, autumnal equinox 



December 21, winter solstice . . 



Our attention is now requisite on the north aspect. The 

 long deep shadows which attend this position on the rising 

 or setting of the sun are strongly characteristic of mountain 

 scenery. In the evening, long before the sun approaches the 

 horizon, the vegetation is plunged in shade, whilst the oppo- 

 site side, perhaps of a fertile mountain valley, still hcs exposed 

 to its warm beams. The direct action of the sun, or the 

 power of solar radiation, over the vegetation is much below the 

 average. Inclined surfaces at an angle of 45° are far beyond 

 what are usually found supporting a rich vegetation. But 

 taking half this amount, it will require a greater altitude than 

 is experienced by us during a portion of the year to relieve it 



