Mr. Editor,--I have a saw-mill which draws thirty-six square inches of water, under thirty feet head. I wish to build another below with only twenty feet head of water. How many square inches aperture will be required to discharge the same quantity in the same time? If some of your correspondents will give me an answer, they will much oblige me. R. C. Navarino, Sept. 7, 1846.

We shall have no occasion to depend on correspondents for the intelligence above required. Thirty-six inches of aperture under thirty feet head, will admit the discharge of 660 cubic feet of water per minute; the velocity of the water being forty-four feet per second. Under twenty feet head the velocity is only thirty-six feet per second, and consequently forty-four inches aperture is required to discharge an equal quantity.

Rule in Hydraulics: (never before published.) To ascertain the velocity of water issuing through an aperture under a given head: Multiply the head in feet by 62, and the square root of the product will show the velocity in feet per second.