PROF. THISELTON DYER ON SCIENTIFIC HORTICULTURE. 15 
planation may be partly due to the downward gravitation of air 
cooled at the surface of the higher ground, and its consequent 
collection in low-lying places and hollows."*^ 
In every department of scientific work it from time to time 
happens that announcements are made which take completely by 
surprise those who know what has really been made out by legiti- 
mate investigation in the subjects upon which they bear, l^ever- 
theless, the outside world always takes them up with more or less 
of uncritical faith. A paper published during last year by 
General Pleasonton, " On the Influence of the Blue Colour of the 
Sky in Developing Animal and Vegetable Life," appears to me to 
have received a great deal more attention than its utter absence 
of any genuine scientific character deserves. Subsequently pre- 
sented to the French Academy, it has been the subject of an 
article by Duchartre in the Bull, de la Soc. Cent. d'Hort. de 
France. This writer points out some of the mistaken scien- 
tific views held by General Pleasonton, but though apparently 
Meteor. Soc. 1872, pp. 100 — 102. The places compared were Denbies, near 
Dorking, 610 feet, and Cobham, six miles distant, 65 feet above the sea-level. 
The maximum temperature of the hill is below that of the valley, the differ, 
ence averaging from 3 to 3| deg. But in times of extreme cold the tempera- 
ture on the hilltop never descends so low as in the valley. Out of forty-three 
occasions when the temperature has been below 25 deg. at both places, the 
average on these occasions upon the hill has been 23 '3 deg. against 18-9 deg. 
at the valley. The temperature has been six times below 10 deg. at the 
lower station, giving an average of 6 deg., while on the hilltop upon the 
same height it averaged 15 deg. On the morning of December 31, 1870, the 
minimum at Cobham was 1*2 deg., at Denbies 14 deg. The popular opinion 
assigned for this difference is that the air of the hill is drier than that of the 
valley ; the observations give no foundation whatever for such an opinion. 
Professor Ragona-scina (Proc. Meteor. Soc. iii.), and Mr. Glaisher (Proc. 
Meteor. Soc. v.), have arrived at the conclusion that through the day the 
higher strata of air are the colder, that towards evening they approach nearly 
to equaUty, but that at night the higher strata of air are the warmer. 
* The stratum of air in contact with ground covered with vegetation 
cools under nocturnal radiation sometimes 8, 10, or 12 deg. Fah. below that 
of the air a few yards above. Thus Humboldt and Bonpland found dew on 
the surface of the ground when the temperature of the air at some little 
distance above it was 70 to 80 deg. The air cooled in this way would 
gravitate into hollows and low-lying places. This seems to explain the fact 
observed by Martins that figs, olives, laurels, etc. , perished in low parts of 
the Botanic Garden at Montpellier, while they escaped a few yards higher in 
equal conditions of shade (Becquerel " Sur les Forets et leur Influence climac- 
terique," Mem. de I'lnstitut 36, pp. 456—469). 
