Hot beds were very popular in Victorian times. Once set up, they can be used to grow salad crops in winter, get a head-start on seed sowing in the spring (by up to a month), and for growing melons and any of the cucurbitaceae family in the summer. A hot bed provides bottom heat, using manure rather than electricity as the heat source, thus speeding up plant growth of seedlings and tender plants.
The heat source: Fresh strawy manure - in a layer 60-90cm deep (after treading). As the manure breaks down, it generates heat. Tread it down well to compact it, ensuring a more even release of heat.
The growing medium: A mixture of top soil and garden compost (ratio of 1:1) - this is placed on top of the manure in a layer 20cm-30cm thick.
The hot bed can be as deep and as wide as you want, as long as the ratio of manure to growing medium is 3:1. If you do decide to make the hot bed deeper, temperatures may rise above the optimum (24C) and plants may be scorched. It can be cooled down by adding water or leaves and garden debris to the mixture. Check temperatures regularly with a thermometer.