OF AGRICULTURE. 125 



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 maraichers, 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 gar- 

 deners begin more and more to make use of 

 portable pipes, or ihermosiphons, provisionally 

 established in the cool frames. This new im- 

 provement becomes of general use, and we have 



