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size, discoverable by their leaves be- two bodies in contact with each other, 

 ginning to decay. i the greater is the rapidity with which 



The roots may either remain in the the progress towards equilibrium goes 

 ground, to be drawn as wanted, or on. This is one reason why a tempera- 

 taken wholly up in autumn when their ture of 32°, with a brisk wind attending 

 leaves decay, and preserved in sand all it, will injure plants to a far greater 

 winter. extent than a temperature many de- 



To save seed. — Let some of the plants grees lower, with a still atmosphere; 

 remain where sown, when they will but it is aided by the operation of ano- 

 shoot up in the spring, and produce ther law of heat, viz., that aeriform bo- 

 plenty of seed in autumn. — Ahercrombie. dies convey it from a cooling body, as 



SCOTCH ASPHODEL. To- a wall or a tree, by an actual change 

 fieldia alpina, in the situation of their own particles. 



SCOTCH LABURNUM. Cytisus al- That portion of the air which is nearest 

 piniis. to the cooling body is expanded, and 



SCOTTIA. Three species. Green- ; becoming specifically lighter, ascends, 

 house evergreen shrubs. Young cut- and is replaced by a colder portion, 

 tings. Sandy loam and peat. i This, in its, turn, becomes heated and 



SCREEN. All cooling is occasioned dilated, and gives place to another 

 either by the heat being conducted from colder portion. And thus the process 

 a body by a colder, which is in contact goes on, until the cooling body is re- 

 with, it, or by radiating from the body i duced to the same temperature as the 

 cooled, though circumstancesaccelerate air. 



or retard the radiation ; and whatever In a still atmosphere, this goes on 

 checks the radiation of heat from a body slowly ; the air in contact with the 

 is a screen, and keeps it warmer. | wall and tree rises very gradually as it 



For example, a thermometer, placed imLiibes warmth from them ; but if there 

 upon a grass plot, exposed to a clear be a brisk wind, a constant current of 

 sky, fell to 35''; but another thermo- air at the lowest temperature then oc- 

 ineter, within a few yards of the pre- curring, is brought in constant contact 

 ceding, but with the radiation of the , with them, and the cooling is rapid, in 

 rays of heat from the grass checked by accordance with the law of equilibrium 

 no other covering than a cambric pocket just noticed. A shelter of netting, or 

 handkerchief, declined no lower than even the sprays of evergreens, are of 

 42'-'. No difference of result occurs the greatest service in preventing the 

 whether the radiating surface be paral- sweeping contact of cold air at such 

 lei or perpendicular to the horizon ; for limes. Snow is a good shelter; it pre- 

 when the mercury in a thermometer, vents heat radiating from plants; pro- 

 hung against an openly exposed wall, tects them from the chilling blasts ; and 

 fell toSS'', another thermometer, against is one of the worst conductors of heat, 

 the same wall, but beneath a web of I have never known the surface of the 

 gauze stretched tightly, at a few inches earth, below a covering of snow, colder 

 distance, indicated a temperature of i than 32'', even when the temperature 

 43°. I of the air above has been 28°. 



These results explain the beneficial ' Strange as it may appear, yet it is 

 operation of apparently such slight nevertheless true, that a screen is more 

 screens to our wall-fruit when in bios- beneficial in preserving the tempera- 

 som. A sheet of canvas or of netting ture of trees, when from three to six 

 prevents the direct radiation of heat inches from them, than when in imme- 

 from the wall; the cooling goes on diate contact with their surfaces. When 

 more slowly, and is not reduced to that a woollen net was suspended four in- 

 of the exterior air at night, before the ches from the wall on which a peach 

 return of day begins to re-elevate the j tree was trained, the thermometer fell 

 external temperature. very slowly, and the lowest degree it 



The colder the body surrounding reached was 38°; when the same screen 

 another hotly, the more rapid the radia- was twelve inches off, it fell to 34°; 

 tion from the latter; for it is a law of and when drawn tightly over the tree, 

 heat that it has a constant tendency to it barely kept above 32^, the tempera- 

 be diffused equally; and the greater ture of the exterior air. When at 

 the diversity of temperature between . twelve inches from the wall, it permit- 



