No method of disposal requires so much and such constant care, although the results show in the improved yield from the farm. This method of broad irrigation is emphatically not the method to be used except where labor is adequate for proper soil cultivation and where this labor can be given constantly and ungrudgingly. Finally, it must be pointed out that care should always be exercised to prevent irrigating sewage coming in direct contact with any of the soil produce. Certainly sewage should not be used to sprinkle over lettuce or celery or strawberries, even if the yield is thereby increased. Undoubtedly any disease germs thus distributed over the fruits and vegetables would, through the antiseptic action of the sunlight and air, soon be destroyed, but the very method of irrigation is repulsive, and the danger, while slight, is sufficient to forbid that method of fertilizing. No statistics, however, are available to show that cows eating sewage-irrigated grass are adversely affected in health, and for years the practice of thus pasturing cows has been carried on in England. For human beings, however, vegetables grown in soil that is separated from the sewage by a foot or more is the safer as well as more æsthetic arrangement. CHAPTER VII ESTIMATES OF COST In order to estimate the cost of installing a sewage-disposal plant and of treating continuously the sewage from any residence, certain fundamental assumptions are always necessary. In the first place, the unit cost of the manual labor which forms so large a part of the total cost of construction must be known for the particular time and place, and perhaps no item in the cost of construction is so important as this. In a great many small installations it may be excluded altogether, since all the hand work required is contributed by the householder at such times as the other work of the place may allow, without any additional cost. In other places, if a money value be placed on such labor, it may be expressed in terms of the cost of a hired man whose rate of wages, paid monthly, in addition to board, would be always less than if wages were paid to day laborers living at their own homes. Again, in the southern part of the country labor may be had for $1.25 a day, whereas in the central portion of the United States it is necessary to pay $1.75 a day, and in the extreme West from $2.00 to $3.00 a day for common labor. Often, too, the working day is of different length in different parts of the country. In the estimates which follow, labor is assumed to cost $1.60 for eight hours’ work, that is, at the rate of twenty cents an hour. If, in adapting the estimates of this chapter to any particular installation, the question of labor may be neglected because of the fact that the householder will himself do all the required work, then the item of labor cost may be eliminated. If other units than those here