492 Plant Case for growing Plants 



periments of Fordyce and Blagden, who were able to remain, 

 for a short time, in a close room raised to the temperature 

 of 212°, or even 260°, Fahr.; and in support of the latter, he 

 appeals to the experience of Mr. King, who accompanied Capt. 

 Back in his late expedition to the arctic regions. That officer 

 states that a difference of 70° or 80°, either from cold to heat, 

 or from heat to cold, did not suspend his usual avocations in 

 the open atmosphere when the air was perfectly still ; but, 

 though the temperature might be 40° higher, if it was accom- 

 panied with a stiff* breeze, he did not stir from home. In 

 like manner, Sir Edward Parry found that a degree of cold 

 sufficient to freeze mercury could be more easily borne when 

 the air was perfectly calm, than when, with a stiff breeze, the 

 temperature was 50° higher. " When the cold was 40^° below 

 freezing on the Fahr. scale," says Mr. Laing in his late Tour 

 in Sweden, " it was quite practicable to prosecute the great cod- 

 fishing in open boats in the Lafoden Isles, within the arctic 

 circle. The calmness of the air, which accompanies this ex- 

 treme cold, is a kind of natural safeguard against its severity, 

 the abstraction of heat from our bodies being then much less 

 rapid. Such a hard winter," he adds, " is considered here a 

 blessing next to a good crop; for the fisherman then gets out to 

 sea, the landsman gets in his timber out of the depths of the 

 forest, and the inhabitants of the most pathless districts obtain 

 their supplies of grain, potatoes, &c, at little cost of transport." 

 (Tour in Sweden, p. 364.) 



The powerful and rapid operation of wind in lowering tem- 

 perature was shown in an experiment of Dr. Heberden, recorded 

 in the Philosophical Transactions for 1826. He suspended a 

 thermometer, previously raised to 100° Fahr., in an atmosphere 

 at 31°, when a strong breeze prevailed, and in about half a 

 minute the mercury fell not less than 48° ; whilst in an atmo- 

 sphere at 30°, but without any perceptible wind, the fall of the 

 mercury, previously raised, as before, to 100°, was only 19° in 

 the same period of time. These facts, which doubtless apply 

 to vegetable as well as to other bodies, due regard being had 

 to differences in their conducting powers, show that degrees of 

 cold may be borne with impunity in an atmosphere that is per- 

 fectly still, which, if accompanied with a brisk wind, would be 

 quite intolerable. That such stillness prevails in the plant cases 

 there can be little reason to doubt ; for though considerable 

 motion may often occur in the air within them from variations 

 in the external heat, yet, as little or none of this air escapes, its 

 temperature, at any given period, must be deemed pretty uniform, 

 and cannot be l'apidly reduced as it is by the frequent contacts 

 and changes of surface which go on in the free motions of an 

 agitated atmosphere. In an atmosphere, too, which is so still, 



