Perishable vegetable productions necessary for the wants of towns must at present be raised in their immediate suburbs; these, however, where they can be transported with a perfectly smooth motion at the rate of twenty miles an hour, will be supplied by the agricultural labourer of more distant points. The population engaged in towns, no longer limited to their narrow streets, and piled story over story in confined habitations, will be free to reside at distances which would now place them far beyond reach of their daily occupations. The salubrity of cities and towns will thus be increased by spreading the population over a larger extent of surface, without incurring the inconvenience of distance. Thus the advantages of the country will be conferred upon the town, and the refinement and civilization of the town will spread their benefits among the rural population.[32] [Footnote 32: Some of the preceding observations appeared in an article contributed by me to the British and Foreign Review.] (101.) The quantity of canal property in these countries gives considerable interest to every inquiry which has for its object the relative advantage of this mode of transport, compared with that of railways, whether worked by horses or by steam-power; and this interest has been greatly increased by the recent extension of railway projects. This is a subject which I shall have occasion, in another work, to examine in all its details; and, therefore, in this place I shall advert to it but very briefly. When a floating body is moved on a liquid, it will suffer a resistance, which will depend partly upon the transverse section of the part immersed, and partly on the speed with which it is moved. It is evident that the quantity of the liquid which it must drive before it will depend upon that transverse section, and the velocity with which it will impel the liquid will depend upon its own speed. Now, so long as the depth of its immersion remains the same, it is demonstrable that the resistance will increase in proportion to the square of the speed; that is, with a double velocity there will be a fourfold resistance, with a triple velocity a ninefold resistance, and so on. Again, if the part immersed should be increased or diminished by any cause, the resistance, on that account alone, will be increased or diminished in the same proportion. From these circumstances it will be apparent that a vessel floating on water, if moved with a certain speed, will require four times the impelling force to carry it forward with double the speed, unless the depth of its immersion be diminished as its speed is increased.