_Answer._ The inside pipe _R_, fig. 41, is usually made of the same diameter as the cylinders, or an inch or two smaller. For the other dimensions there are no established rules, excepting for the height of the top of the chimney above the rail, which is usually from 14 to 15 feet. The outsides of smoke-stacks are made of sheet iron, but the upper part is now sometimes made of cast iron, so as to withstand the abrasion of the sparks and cinders longer than sheet iron will. For very warm and damp climates, the outsides of smoke-stacks are sometimes made of copper to resist corrosion, which is very destructive to all iron structures in those countries. The wire netting is made of iron or steel wire from ¹⁄₁₀ to ¹⁄₃₀ of an inch in diameter, and with from 3 to 4 meshes to the inch. QUESTION 122. _What is the pipe N, fig. 41, intended for?_ _Answer._ It is intended to conduct the exhaust steam and a portion of the smoke from the bottom of the smoke-box, where the steam escapes, to the base of the smoke-stack. In nearly all European engines, the exhaust steam escapes at the top of the smoke-box just below the aperture of the smoke-stack. There is, however, often difficulty in equalizing the draft in the tubes, that is, to get an equal amount of smoke to pass through all of them. By the use of the pipe _P_, called the _inside pipe_ or _petticoat pipe_, it is thought that the draft in the tubes can be equalized much better, as a part of the smoke is drawn out of the smoke-box at the top and part at the bottom of the smoke-box. The pipe is usually made in two parts, which slide into each other like a telescope. The distance of the upper end from the top of the smoke-box and that of the lower end from the bottom can thus be increased or diminished, and if the draft is greater through the upper tubes than through the lower ones, or _vice versa_, it can be regulated or equalized by simply raising or lowering the top or bottom of the petticoat pipe. Sometimes this pipe is made with openings and a kind of deflectors over them between the two ends. It is then called a _flounced petticoat pipe_, for obvious reasons. No exact theory can be stated regarding the proportions of these pipes, or the results effected by them, which can be determined only by practical experience. Some more accurate knowledge concerning them is, however, much needed. PART VIII. THE BOILER ATTACHMENTS. QUESTION 123. _How is water supplied to the boiler to replace that which is converted into steam?_ _Answer._ It is usually forced into the boiler against the steam pressure by force pumps, but another instrument called an _injector_ is now much used. QUESTION 124. _What is the form of construction and principle of the operation of such force pumps?_