314 PROTECTION OF PLANT-HOUSES DURING COLD NIGHTS. 



grass, which was thus shielded from the sky, was, upon many 

 nights afterwards, examined by me, and was always found 

 higher than the neighboring grass which was uncovered, if this 

 was colder than the air. When the difference in temperature 

 between the air several feet above the ground and the unshel- 

 tered grass did not exceed 5°, the sheltered grass was about as 

 warm as the air. If that difference, however, exceeded 5°, the 

 air was found to be somewhat warmer than the sheltered grass. 

 Thus, upon one night, when fully exposed grass was 11° colder 

 than the air, the latter was 3° warmer than the sheltered grass, 

 And the same difference existed on another night, when the air 

 was 14° warmer than the exposed grass. One reason for this 

 difference, no doubt, was, that the air which passed from the 

 exposed grass, by which it had been very much cooled, had 

 passed through that under the handkerchief, and deprived the 

 latter of part of its heat. Another reason might be given, — 

 that the handkerchief, from being made colder than the atmos- 

 phere, by the radiation of its upper surface to the heavens, would 

 remit somewhat less to the grass beneath, than what it received 

 from that substance. But still, as the sheltered grass, notwith- 

 standing these drawbacks, was, upon one night, as may be seen 

 from the preceding account, S°, and upon another, 11°, warmer 

 than grass freely exposed to the sky, a sufficient reason was 

 now obtained for the utility of a very slight covering, to protect 

 plants from the influence of frost or external cold."* 



* As the elevation of temperature, induced by the heat of summer, is 

 essential to the full exertion of the energies of the vital principle, so the 

 depression of temperature, consequent upon intense cold nights, has been 

 thought to suspend the exertion of the vital energies altogether. But this 

 opinion is evidently founded on a mistake, as is proved by the example 

 of such plants as protrude their leaves and flowers in the winter season 

 only, as well as by the dissection of the yet unfolded bud, at diflerent 

 periods of the winter, which proves regular and progressive develop- 

 ment ; even in the case of such plants as protrude their leaves and 

 flowers in the spring and summer, and in which, as we have said, there 

 is a gradual, regular, and incipient development of parts, from the time 

 of the bud's first appearance, till its ultimate opening in the spring. The 

 sap, it is true, flows much less freely, but it is not wholly stopped. Du 

 Hamel planted some young trees in the autumn, cutting off all the 



