(16) F. M. M. asks: 1. How large must an engine be to run a boat 12½ feet wide, 75 feet long, drawing 4 feet of water, at the rate of 30 miles per hour, on a river or bay where the surface is smooth? A. We have some doubts as to whether these conditions could be fulfilled. 2. Do steamboats on the ocean use salt water in their boilers for steam, or do they carry fresh water? A. They ordinarily have surface condensers, so that the water of condensation is returned to the boilers. (17) E. S. N. says: Please give your ideas as to how much water an engine 18 inches in diameter by 22 inches stroke, running at 145 revolutions per minute, at 80 lbs. steam, cutting off at about 18 inches, will require. We furnished an injector for one of the above dimensions, capable of throwing 900 gallons per hour. It was found to be insufficient, and I went to the mill to discover the cause, if possible. The manufacturers of the injector thought it ought to be large enough, and so did we. I found everything set up properly, and the piston and valve were evidently in good order. I finally measured the capacity of the tank which supplied the injector, and found that it drew 960 gallons per hour. A. We do not think the data are sufficient for an accurate calculation. It is possible, however, that some of our readers have made experiments on similar engines, and can give some useful information. (18) T. W. asks: What size of breast water wheel, with a fall of 2 feet water, would it require to produce the same power as an overshot wheel of 4 feet diameter, 18 inches face, with a fall of 5 feet water? A. If the breast wheel gave the same efficiency as the other, it would require a face about 2½ times as wide. (19) A. K. says: A. asserts that if a small and a large boiler be set side by side and connected with the top gauge cock of the two boilers, level, when they are first filled with water, and then steam is raised, that the water will not remain the same, that the pressure will be greater in the larger boiler, and consequently will force the water into the smaller one. B. says that the water will always remain the same as long as the boilers are connected; that the pressure on the water will be the same in both boilers, and therefore the water will always assume the same level in each. Which is right? A. The pressures sometimes vary in two boilers connected in this way; and they should be set in such a way that the water cannot be forced from one into the other under any circumstances.