234 NOTICE OF A VISIT TO PITMASTON. 



air flows into the latter by the apertures at top, whilst at the 

 same time the colder air is withdrawn at bottom to an equal ex- 

 tent. Enclosed by non-conducting materials as regards heat, 

 the air in the room would long retain its warmth if entirely shut 

 up. But its store is gradually transferred to the vinery, by the 

 communications existing between them, whenever the tempera- 

 ture of the vinery falls below that of the room. A very simple 

 experiment, the principle of which, I believe, has been often 

 familiarly explained, may be referred to as affording the easiest 

 possible illustration of the mode of action resulting from the 

 above arrangement. Let two close rooms be unequally heated ; 

 partially open a door between them, and in the opening place a 

 lighted candle on the ground, whilst another is held near the 

 top ; their flames will be deflexed in contrary directions. The 

 lower one will indicate the direction of the current of colder and 

 consequently heavier air, from the cold into the hot room ; and 

 the one at top will be seen urged in the opposite direction by the 

 egress of warm air from the hot into the cold, the action conti- 

 nuing till an equilibrium is established in the air of both rooms, 

 which can only be the case when both acquire an equal tem- 

 perature. 



The benefit to be derived from the above principle may be 

 approximately calculated in the case of a vinery, or other forcing- 

 house. Supposing the room to contain as many cubic feet of air 

 as is contained in the vinery with which it is in communication, 

 say 6000 cubic feet in each ; supposing also that by sun heat, the 

 air in the vinery rises as much above 70° as will be sufficient to 

 render the whole of the air in both compartments of that tem- 

 perature, and tiiat the cold at night would reduce the 6000 cubic 

 feet of air under glass from 70° to 40°, it follows that 12,000 cubic 

 feet, the quantity of air in both compartments, will only be re- 

 duced half as much, that is to 55°. Again, if the air in a green- 

 house was liable to be cooled down from 50° to 30°, then, in com- 

 munication with a quantity of air equal to that contained in the 

 green-house alone, and to be also reduced from 50°, the minimum 

 of the green-house would be 40° instead of 30°; and thus, with- 

 out fire-heat, the plants would be in a comfortable medium. In 

 practice, however, it will doubtless be found that the minimum 

 will be somewhat lower than is indicated by the above calcula- 

 tions, because the inertia of the air interferes with the otherwise 

 free circulation between the respective compartments. 



Where the surface of glass is large in comparison with the 

 quantity of air in the interior, the plants are apt to suflTer from 

 vicissitudes of temperature, arising from the rapidity with which 

 a small body of air under glass is heated and cooled. A com- 

 munication with a larger body of air in some adjoining compart- 



