There prevails, however, with regard to market-gardening, a misunderstanding which it would be well to remove. It is generally supposed that what chiefly attracts market-gardening to the great centres of population is the market. It must have been so; and so it may be still, but to some extent only. A great number of the Paris _maraîchers_, even of those who have their gardens within the walls of the city and whose main crop consists of vegetables in season, export the whole of their produce to England. What chiefly attracts the gardener to the great cities is stable manure; and this is not wanted so much for increasing the richness of the soil--one-tenth part of the manure used by the French gardeners would do for that purpose--but for keeping the soil at a certain temperature. Early vegetables pay best, and in order to obtain early produce not only the air but the soil as well must be warmed; and this is done by putting great quantities of properly mixed manure into the soil; its fermentation heats it. But it is evident that with the present development of industrial skill, the heating of the soil could be obtained more economically and more easily by hot-water pipes. Consequently, the French gardeners begin more and more to make use of portable pipes, or _thermosiphons_, provisionally established in the cool frames. This new improvement becomes of general use, and we have the authority of Barral’s _Dictionnaire d’Agriculture_ to affirm that it gives excellent results. Under this system stable manure is used mainly for producing loam.[59] As to the different degrees of fertility of the soil--always the stumbling-block of those who write about agriculture--the fact is that in market-gardening the soil is always _made_, whatever it originally may have been. Consequently--we are told by Prof. Dybowski, in the article “Maraîchers” in Barral’s _Dictionnaire d’Agriculture_--it is now a usual stipulation of the renting contracts of the Paris _maraîchers_ that the gardener may carry away his soil, down to a certain depth, when he quits his tenancy. He himself makes it, and when he moves to another plot he carts his soil away, together with his frames, his water-pipes, and his other belongings.[60]