In all hotbeds the underlying principle is the same:  They are right-angled boxes covered with glass panes set in movable frames and placed over heated excavations.  The bed may be of any size or shape, but the standard one is six feet wide, since the stock glass frames are usually six feet long by three feet wide.  You can have any length needed to supply your requirements.  “Tomato Culture,” by A. J. Root, tells us that the cheapest plan is to get some old planks, broken brickbats or stone, and piece together a box-like affair in proper shape:  to provide drainage, the front should be at least ten inches above the ground and the rear fourteen inches.  A hotbed knocked together in this way is all right to start with, if you cannot do any better, but will last only two or three seasons.  For a permanent bed, probably the best way is to make cement walls extending to the bottom of the manure.  The bed ought to face south or southeast and be well protected on the north.  It should be banked all around with earth or straw to keep out the cold, and mats or shutters should be provided for extra cold weather.  The best material for heating the bed and the most easily obtained, is fresh horse manure in which there is a quantity of straw or litter.  This will give out a slow, moist heat and will not burn out before the crops or the plants mature.  Get all the manure you need at one time.  Pile it in a dry place and let it ferment; every few days work the pile over thoroughly with a dung fork; sometimes two turnings of the manure are enough, but it is better to let it stand and heat three or four times.

“You can make a hotbed also on top of the ground without any excavation.  Spread a layer of manure evenly one foot in depth and large enough to extend around the frame three feet each way.  Pack this down well, especially around the edge, put on a second and third layer until you have a well-trodden and compact bed of manure at least two and one half feet in depth.  Place the frame in the center of this bed and press it down well.”  A two-inch layer of decayed leaves, cut straw, or corn fodder, spread over the manure in the frame and well packed down, will help to retain the heat.  Ventilate the bed every day to allow steam and ammonia fumes to pass off.

“The soil inside should be equal parts of garden loam and well-rotted barnyard manure.  Tramp well the first layer of three inches.  To make it entirely safe for the plant seeds in the hotbed, add another layer of the same depth.  Use no water with garden loam and manure if you can possibly help it.”

“Before sowing any seeds put a thermometer in the bed three inches deep in the soil.  If it runs over 80 degrees Fahrenheit, do not sow.  If below 55 degrees it is too cold; you will have to fork it over and add more manure.  If the bed gets too hot, you can ventilate it with a sharp stick by thrusting it down into the soil.”