142 Forcing-Pit heated hy hot Water, 



over this should be placed a layer of turf with its grassy side 

 downwards, where it is intended to cultivate melons or cucumbers ; 

 for pines or plants in pots, fine gravel or coal-ashes should be 

 employed, in which the pots might be plunged. The pit is now 

 complete. When the fire is kindled and the pipes heated, the 

 air in the flue or chamber c being heated, will rise to the top of 

 the flue, and imparting some of its heat to the slate above, will 

 proceed up the cross flue and pilaster p, and be poured out into 

 the pit ; its place below being supplied by the cooler air from 

 the front, or lower, side, descending by the hollow pilaster in front 

 to be heated in its turn by the pipes below. The slate, being an 

 excellent conductor, will transmit abundance of bottom heat. 

 It will, of course, take a few days, after the apparatus is set at 

 work, before the mass of rubbish, &c., becomes thoroughly 

 warmed, after which it will be subject to little variation. 



All moisture in the pit, produced by watering or otherwise, will 

 gradually sink down to the central flue, towards which the 

 floor of the pit should slope from both sides. On arriving at 

 this flue the moisture will be immediately taken up by the air 

 heated by the pipes, and, being brought up as it circulates, will 

 be deposited by the cooling air in the pit above. The arrows 

 in^. 29. indicate the course of the circulating air. 



By this arrangement, it will be perceived, three things are 

 attained: bottom heat; top heat, with an atmosphere constantly 

 moist; and, finally, a constant circulation of air. Unless I am much 

 mistaken, supposing the pit to be 6 ft. wide and 3 ft. high at back, 

 and 1 ft. 3 in. in front, above the mould, the whole atmosphere 

 of such a pit would pass through the flues once in every two 

 minutes, when the apparatus was in full work, thus producing a 

 constant and vigorous circulation. Further, if it be desired to 

 change a portion of the air continually, small apertures to the 

 outward air may be made in the front descending flues ; a small 

 portion of fresh air will then pour in, continually mingling with 

 the descending air, and ascending heated into the pit, supplying 

 the place of that which will escape through unputtied laps, and 

 various crevices. The internal areas of the pilaster flues should 

 not be less than 6 in. square, which they may be according to 

 the proposed plan ; and the cross flues and the apertures into 

 the pit must have the same area. If it be found that, with 

 this area or aperture, the heat produced by the pipes is brought 

 up too rapidly, not leaving sufficient bottom heat, and overheating 

 the top, these apertures may be diminished. If they be too 

 small, an inequality will arise between the temperature of the 

 back and front of the pit, caused by an excessive difference of 

 the ascending and descending air. The total heat brought up 

 will be the same, for, as the heat increases, the velocity with 

 which the air will ascend will increase also ; but, if apertures of 



