Certain communications, lately received, in reference tc boiler construction, lead to the inference that the importance of securing proper circulation in boilers is imperfectly comprehended on the part of some of our readers. In order tc make this subject comprehensible to those unskilled in boilei construction, we will briefly state what will take place when water is boiled. First, the heating of water to 212 Fah., its boiling point under atmospheric piessure, at the sea level, is not synonymous with boiling, for if water be taken to lower levels, as in deep mines, it will not boil, on account of increased atmospheric pressure, till a higher temperature has been reached. In all steam, boilers, when condensation is not employed to remove atmospheric pressure from the exhaust, a greater pressure than that of the atmosphere must be maintained, to give an efiective pressure in the cylinder. It follows that in such boilers the temperature at which steam escapes from the surface of the water in such force as to cause ebullition or boiling must be higher than 212 Fah., by an amount corresponding to the pressure maintained. The violence with which the boilijig of water takes place, when the proper temperature is attained, depends not only very much upon the shape of the containing vessel, but in an almost equal degree upon the point at which the heat is applied to it. Water, contained in a long, straight, and narrow tube, closed at the bottom, may be quietly and entirely evaporated if the tube be held in a gas flame, obliquely, so that the heat shall be applied successively to the uppermost strata as evaporation proceeds. If, on the contrary, the tube be so held that the bottom stratum receives the heat first, the water will either be thrown out of the tube, or the bottom will be blown out. But a tube might be so formed and adjusted that heat applied at the highest point would also force out the water. A tube, bent into the form of the letter U, filled with and having its lower ends immersed in water, would be such a tube. All violent ebullition in a boiler not only tends to render it unsafe, but also ueconomical from foaming or priming. If we were addressing our remarks to experts, we should not need to say that the amount of water required to feed a boiler is no index of its evaporative power. Water thrown out unconverted into steam is not evaporated. It performs no work in the cylinder of the steam engine ijnless converted into steam by what is called superheating. As water does not boil until steam is generated in sufficient volume, and as this event depends, all other things being equal, upon the amount of surface pressure, it follows that in high-pressure boilers a much greater increment of heat must first be imparted to the water than in low-pressure boilers,and that priming is more likely to occur in them on account of the higher temperature and greater activity of the steam. Previous to boiling, the contained water circulates quietly, the heated particles rising as fast as heated, and giving place to colder ones, until the temperature of the mass is raised to the point at which steam is rapidly generated. When this takes place, every particle of water converted into steam occupies some three hundred times the space which it filled in the liquid state, so that, in escaping to and through the surface, it requires a veiy much larger and freer passage thai before. If this is not provided, the water will be violently upheaved, and dashed about, spray will be produced, and wet steam become a certain result. Such was the case in the now discarded suspended tubes employed some years since in the earlier forms of portable en- gines. These tubes, suspended in the fire box, made no adequate provision for circulation, and, of course, were found to be ineflicient as well as dangerous. The form of boiler which, of course, will give the freest circulation of any, is the plain cylinder, without tubes or flues. But such boilers give only a small heating surface in proportion to the amount of water they carry, and on other accounts are not economical. It will be found generally better to pass the fire through tubes or flues surrounded by water, than the converse. And it is also necessary that a proper distance between such flues should be maintained. In any tubular boilei the tubes ought not to be too small. It is safer and more economical to err on the other side, if slight error is unavoidable, and to provide for a free and perfect circulation. Circumstances which determine the proper proportions, are very numerous, and ought to be well considered, in the construction of every flue or tubular boiler. He who, without posting himself upon the results of modern experience and investigation, expects to achieve much success, will be doomed to disappointment.