344 VIII. ALTITUDE OF SPECIES. 



tbe altitude is added, in accordance with the scales or rates of decrease 

 hefore suggested, on pages 162 to 166. But in looking at the figures 

 for the two scales, Fahrenheit and centigrade, it must still be recol- 

 lected that the rales of decrease were expressly adapted to the two 

 diflferent thermometers, and thus could not be rendered exactly corre- 

 spondent one with another. (Page 214). Consequently, the tempera- 

 tures expressed by the decimal figures printed on the same lines in the 

 list here under notice, as being the temperatures for the given altitudes, 

 according to the two thermometrical scales, will not be found exactly 

 equal one with another ; the difference between them being very slight 

 at first, and becoming wider as the altitude increases. At the lowest 

 altitudes (say, below 250 yards, in the latitude of the Grampian moun- 

 tains) the Fahrenheit scale is slightly in excess over the centigrade. At 

 the highest altitudes, their slight differences are more than reversed ; 

 the centigrade being there in excess over the Fahrenheit scale. These 

 differences may be rendered more obvious at a glance, by printing the 

 true correspondence or equality, along with the figures from the two 

 scales, as the latter are applied in this volume. In latitude 57, from 

 the sea-level upwards, we may trace the following series of figures, on 

 looking to that portion of the preceding lists which relates to the upper 

 limits of plants on the Grampian mountains, page 323 : — 



yards. 47 fahr. 8.20 cent. 8.33 true corresp. 



7.20 7.22 



6 20 6. II 



5.20 5.00 



4.20 3.89 



3.20 2.78 



2.20 1.67 



1.20 0.56 



So that, at the highest altitude, 1400 yards, the inequahty between the 

 two scales, arising out of their different rates of decrease, attains to 0.64, 

 or nearly two-thirds of a degree, — centigrade in excess over Fahrenheit. 

 It is impossible yet to say confidently, which of these two is the more 

 accurate ; though probability seems in favour of the centigrade, as more 

 nearly correct in its rate of decrease upwards. Indeed, the Fahrenheit 

 scale is added only for the convenience of those phyto-geographers who 

 are unaccustomed to use the other thermometer, and to whom its figures 

 consequently express the temperatures less clearly or less significantly. 

 A remark is incorrectly applied on page 158, immediately underneath 



