To return from our digression. It is necessary, in order to a correct apprehension of the work which our drains have to perform, to form a correct opinion as to how much of the surplus moisture in our field is due to each of the three causes to which we have referred--to wit, rain-water, which falls upon it; springs, which burst up from below; and water of pressure, stagnant in, or slowly percolating through it. The rain-tables will give us information as to the first; but as to the others, we must form our opinion from the structure of the earth around us, and observation upon the field itself, by its natural phenomena and by opening test-holes and experimental ditches. Having gained accurate knowledge of the sources of moisture, we may then be able to form a correct opinion whether our land requires drainage, and of the aid which Nature requires to carry off the surplus water. WHAT LANDS REQUIRE DRAINAGE? The more one studies the subject of drainage, the less inclined will he be to deal in general statements. "Do you think it is profitable to underdrain land?" is a question a thousand times asked, and yet is a question that admits of no direct general answer. Is it profitable to fence land? is it profitable to plow land? are questions of much the same character. The answers to them all depend upon circumstances. There is land that may be profitably drained, and fenced, and plowed, and there is a great deal that had better be let alone. Whether draining is profitable or not, depends on the value and character of the land in question, as well as on its condition as to water. Where good land is worth one hundred dollars an acre, it might be profitably drained; when, if the same land were worth but the Government price of $1.25 an acre, it might be better to make a new purchase in the neighborhood, than to expend ten times its value on a tract that cannot be worth the cost of the operation. Drainage is an expensive operation, requiring much labor and capital, and not to be thought of in a pioneer settlement by individual emigrants. It comes after clearing, after the building of log-houses and mills, and schoolhouses, and churches, and roads, when capital and labor are abundant, and when the good lands, nature-drained, have been all taken up. And, again, whether drainage is profitable, depends not only on the value, but on the character of the soil as to productiveness when drained. There is much land that would be improved by drainage, that cannot be profitably drained. It would improve almost any land in New England to apply to it a hundred loads of stable manure to the acre; but whether such application would be profitable, must depend upon the returns to be derived from it. Horace Greeley, who has his perceptions of common affairs, and especially of all that relates to progress, wide awake, said, in an address at Peekskill, N. Y.: