OF ATMOSPHEEICAL TEMPEEATUEE. 141 



a sudden thaw, which, as gardeners well know,* 

 causes the death of plants which, if slowly thawed, 

 would sustain no inconvenience from the low temper- 

 ature to which they had been exposed.f It is proba- 

 ble, as I have elsewhere endeavoured to show, that 

 this singular effect may be accounted for as follows : — 

 "In such cases, it may be supposed that the air, 

 forced into parts not intended to contain it, is 

 expanded violently, and thus increases the disturb- 



* See Hori Trans., iiL 43. 



f [In the northern and eastern sections of the TJnion many beau- 

 tiful shrubs and plants, -w^hioh are the ornaments of our gardens in 

 summer, but perish if exposed to the rigorous cold of winter, are 

 easily preserved upon this principle. The first impulse of tjie 

 novice in gardening is to place such half-hardy plants (as the more 

 delicate China Roses, Carnations, <Ste.) in some warm sheltered spot, 

 open to the genial rays of the sun in winter : a practice invariably 

 followed by their destruction. Our sun, even in mid- winter, often 

 shines with great brightness, and the thawing and distension of the 

 tissue of tender plants which therefore follows causes certain death. 

 If, on the contrary, the same species are placed in a cool shaded 

 aspect, or, what is preferable, if they are shielded from the sun by 

 a loose covering of straw, mats, or even boards, and thus kept irom 

 thawing except in the most gradual manner, they will be found to 

 have sustained no injury whatever. We have seen a large num- 

 ber of the choicest Camellias preserved without any artificial heat 

 through a cold winter, when the mercury ranged below zero for seve- 

 ral weeks, simply by covering them with a common glazed frame, 

 well clothed with mats to. exclude the direct rays of the sun, or pre- 

 vent sudden variations of temperature. For the same reason, or- 

 chards of Peach trees in the middle States, on the cold north sides 

 of hills, are often more vigorous and of greater longevity than those 

 in a full southern aspect : the heat of our summers being sufficient to 

 ripen their fruit and wood in such situations, while they are thus 

 secured from the evils of great and sudden changes of temperature 

 in winter. A. J. D.] 



