The following table, taken from Hood's work on hot-water heating, may be given as showing the length of 4-in. piping required to heat 1000 cub. ft. of air per minute from 45° to 90° F., the temperature of the pipes being 200° F.

 Temperature of External Air. Temperature at which the House is to be kept. 45° 50° 55° 60° 65° 70° 75° 80° 85° 90° Number of Feet of 4-in. Pipe. 10° 126 150 174 200 229 259 292 328 367 409 20° 91 112 135 160 187 216 247 281 318 358 30° 54 75- 97 120 145 173 202 234 269 307 32° 47 67 89 112 137 164 193 225 259 296 40° 18 37 58 80 104 129 157 187 220 255 50° - - - 19 40 62 86 112 140 171 204

If a house containing 10,000 cub. ft. of air is to be kept at a temperature of 70° F., the external air being at 32° (freezing-point), the amount of piping required is found thus: Go down the column under 70° and find the figures opposite the given temperature of the external air, that is 32°. The figures 164 stand opposite this and beneath the 70°. Multiply 164 by 10, and the result 1640 represents the number of feet of 4-in piping-according to Hood's method. This, however, will scarcely do for horticultural purposes, as no one would dream of heating his hot-water pipes up to 200° F. - only twelve degrees below boiling-point. And, moreover, the length of piping cannot be varied at will, in accordance with the fluctuations of the external air. The quantity of piping is really regulated according to whether a structure is to be treated as a greenhouse or a hothouse, the latter requiring about twice as much piping as the former. Taking a house 100 ft. long, 12 ft. wide, and 8 ft. high to the ridge board with walls to the eaves 3 ft. high, we get a house with about 9000 ft. cubic capacity. If used as a greenhouse with a minimum winter temperature of 45° F., about 500 ft. of 4-in. piping will be sufficient in the usual way, but an extra 200 ft., making 700 ft. altogether, would maintain a temperature at a minimum of 50° to 55°. In a similar house, 1000 to 1200 ft. of 4-in. piping would maintain a stove temperature during the winter months without heating the pipes to more than 100° F.

Heating horticultural structures by steam is practised in America, where climatic conditions are different, but it is not likely to be adopted in Britain.