IV. CLIMATE. 171 



cessively killed off by the increasing severity of the frosts 

 of autumn or winter, if left out unsheltered. And during 

 winters of unusual severity even some of the native spe- 

 cies are injured or quite killed by very low temperatures. 

 It is less easy to trace any similarly destructive influence 

 from unusually high temperatures ; because these being 

 almost constantly accompanied by excessive dryness, the 

 results may be attributed to either or both conditions. 

 But the well-known fact of extreme temperatures being 

 fatal to plants, either directly or indirectly, shows the 

 importance of ascertaining the ordinary and extra- 

 ordinary extremes which are experienced in any country 

 or locality under consideration botanically. 



According to Luke Howard's registers for the ten years 

 following 1806, in the neighbourhood of London, the 

 extremes observed by him were 96 and — 5, respectively 

 in 1808 and 1816 ; thus giving a total range of 101 de- 

 gi'ees. Howard alludes also to a temperature of — 6.5, 

 noted by another observer in 1796 ; thus adding one 

 degree and a half to the full range observed by himself. 

 During the twenty years following 1837, at Thames 

 Ditton, a dozen miles south from London, the observed 

 extremes were 90 and — 8, respectively in 1846 and 1838 ; 

 the greater cold and lesser heat, than those noted' by 

 Howard, thus giving together the reduced range of 98 

 only. Combining the highest temperature recorded by 

 Howard, with the lowest observed at Thames Ditton, the 

 extreme range is raised to 104. As • recorded by the 

 same Author, in his volumes on ' The Climate of Lon- 

 don,' the extreme temperatures during each of the ten 

 successive years were these : — 



Max. 87, 96, 83, 85, 88, 78, 85, 91, 80, 81. 



Miu. 13, 12, 18, 10, 14, 18, 19, 8, 17, -5. 



Thus, a temperature below zero of Fahrenheit occurred 



