seventh smaller cylinder, over the sixth, under the fifth, over the fourth, under the third, over the second, and partly over, and partly under the first; when it passes into the chimney. The wall before mentioned, which divides. the furnace longitudinally, answers the double purpose of lengthening the course which the flame and heated air have to traverse, giving off heat to the boiler in their passage, and of securing from being destroyed by the fire the flanges or other joinings employed to unite the smaller tubes to the main cylinder. The ends of the smaller cylindric tubes rest on the brick-work which forms the sides of the furnace, and one end of each of them is furnished with a cover, secured in its place by screws, or any other adequate means, but which can be taken off at pleasure, to allow the tubes to be freed from time to time from any incrustation or sediment which may be deposited in them. To any convenient part of the main cylinder a tube is affixed, to convey the steam to the steam engine, or to any vessel intended to be heated by means of steam.

"When very high temperatures are not to be employed, the kind of boiler just described is found to answer very well; but where the utmost force of the fire is desirable, Mr. Woolf, for a reason which shall be afterwards mentioned, combines the parts in a manner somewhat different, though the same in principle.

"In fig. 2, A is the main cylinder crossing the smaller cylinders a a a, half way between their middles and ends, but not joined to any of them, excepting the middle one at the points at which it crosses them. It is put in this place that it may come over that part of the furnace, SSS, fig. 1, through which the flame first passes, and receive its direct action, which it does over nearly a half of its surface, as may be seen by looking at the vertical section, ASS, fig. 1. The smaller cylinders have a communication with the main cylinder in the following manner :-Three cylinders, CC C, are placed parallel to the main cylinder, A, over the part of the furnace by which the flame returns, in such a manner that each of the cylinders, CCC, takes in three of the smaller cylinders, a a a, being united to

and connected with them. The cylinders, CCC, have a direct communication with the main cylinder, A, by the pipes or tubes, PPP, as may be better seen by the cross vertical section, fig. 1. The three tubes CC C, are preferred to one long tube, to prevent any derangement taking place in the furnace or in the tubes, by the expansion or contraction occasioned by changes of temperature, which is more considerable in one tube of the whole length of the furnace, than when divided into three portions; and it is for the same reason that the tube A is not made to communicate directly with the smaller tubes, a a a, but mediately, by means of the tubes marked C and P.N. B. The two outermost of the tubes marked P, instead of going parallel to the middle tube, P, may be both inclined towards it, so as to join the cylinder A near the middle; or any other direction may be given to them, to prevent derangement by expansion.

"The tubes C and a are kept from separating by bolts from the inside of a passing through the top of C, where they are secured by nuts screwed on to them, (see fig. 3.); and these parts of C are so contrived, that by taking off any of the nuts a cover may be removed, and a hole presented large enough to admit a man's hand into C, to clean it out.

"Fig. 3. is a longitudinal vertical section of the furnace, through the centre, showing the course which the flame and heated air are forced to take. The first three small cylinders are completely surrounded with flame, being directly over the fire: the flame is stopped by the brickwork, W, over the fifth, and forced to pass under it, and then over the sixth, where it again meets with an interruption, which forces it to go under the seventh, and over the eight; it then turns round the end of the longitudinal wall which divides the furnace, and passes over the eighth smaller cylinder, under the seventh, and so on, alternately over and under the other tubes, till it reaches the chimney. The wall that divides the furnace may be seen in fig. 2. "To secure a free communication between the different parts of the boiler, the three tubes of the middle cylinder,

C, are connected with those of the two exterior C's, by two pipes, o o. The other ends of the tubes, a a a, are each fitted with a cover properly secured and bolted, but which can be taken off occasionally, to clean out the boiler.

"In working with such boilers, the water carried off by evaporation is replaced by water forced in by the usual means; and the steam generated is carried to the place intended by means of tubes connected with the upper part of the cylinder A.

"It may not be improper," says Mr. Woolf, " to call the attention of those who may hereafter wish to construct such apparatus, to one circumstance; namely, that in every case the tubes composing the boiler should be so combined and arranged, and the furnace so constructed, as to make the fire, the flame, and the heated air, to act around, over, and among the tubes, embracing the largest possible quantity of their surface. It must be obvious to any one that the tubes may be made of any kind of metal; but I prefer castiron as the most convenient. The size of the tubes may be varied: but, in every case, care should be taken not to make their diameter too great; and it must be remembered that the larger the diameter of any single tube in such a boiler, the stronger it must be made in proportion, to enable it to bear the same expansive force as the smaller cylinders. It is not essential, however, to my invention that the tubes should be of different sizes; but I prefer that the upper cylinders, especially the one which I call the main cylinder, should be larger than the lower ones, it being the reservoir, as it were, into which the lower ones send the steam, to be thence conveyed away by the steam pipe or pipes. The following general direction may be given respecting the quantity of water to be kept in a boiler in my construction: it ought always to fill not only the lower tubes, but the main cylinder, A, and the cylinder, C, to about half their diameter, that is, as high as the fire is allowed to reach, and in no case ought it to be allowed to get so low as not to keep full the necks or branches which join the smaller cylinders, marked with the letter a, to the cylinders A or C; for the fire is only beneficially

employed when applied, through the medium of the interposed metal, to water, to convert it into steam: that is, the purpose of my boiler would, in some measure, be defeated, if any of the parts of the tubes exposed to the direct action of the fire, should present in their interior a surface of steam instead of water, to receive the transmitted heat which must, more or less, be the case, if the lower tubes, and even a part of the upper, be not kept filled with the liquid.

"As to the construction of the furnaces, though that must be obvious from the drawings, it may not be improper here to remark, that they should always be so built as to give a long and waving course to the flame and heated air, or vapour, forcing them the more effectually to strike against the sides of the tubes which compose the boiler, and so to give out a large portion of their heat, before they reach the chimney: unless this be attended to, there will be a much greater waste of fuel than necessary; and the beat, communicated to the contents of the boiler, will be less from a given quantity of fuel.

"My invention is not only applicable to all the uses to which the boilers in common use are generally applied, but to all of them with much better effect than the latter; and can, besides, be applied to purposes in which boilers, constructed as they have hitherto been, would be of little or no use. The working of all kinds of steam engines is one important application of my invention; for the steam may be raised, in a boiler constructed in the manner before described, to such a temperature, and consequently to such an expansive force, as to work an engine even without condensing the steam, by simply allowing it to escape into the atmosphere after it has done its office, as proposed by Mr. James Watt, in the specification of his patent, dated January 5, 1769, whence, he says, engines may be worked by the force of steam only, by discharging the steam into the open air. In all cases where it is desirable to heat or boil water, or other fluids and substances, without the direct application of fire to the vessel or vessels containing them, which in such cases become secondary boilers, the