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later Firmenich device we have a compactness of structure akin to that of Stirling, and also access for cleaning by the same number of manholes, and in the same way. The proofs show that Fir. menich boilers, built substantially on the lines of the first patent, have been in highly successful operation at the American Cutlery Works in Chicago for upwards of 16 years; that they have not required any repairs of moment in 10 years; that no scale forms on the tubes, and scrapers are not required to clean them; that they are washed with hose, and entrance is had through a manhole at the end of each drum. But the development did not cease with these patents. In 1880 we find the Fowler boiler of patent No. 233,228. While it has faulty features, yet, on the whole, it containsevidence of advance. In itstwo upper and two lower drums we have a departure from the compact triangular construction first shown, but we note for the first time several features which were afterwards modified and carried forward in the Stirling. The cylinders are placed at right angles to the course of the flames, as the patent says, “to cause the flames and gases to break up and pass around among the tubes." We also find the water tubes are bent at both ends, for the express purpose of "spreading out and afford ing room for the action of the products of combustion,” “of en. tering the shells at the proper angle," and to counteract the “injurious effects of the expansion and contraction of the tubes under variations of temperature.” We also note for the first time the forward pitch of the water tubes and upper cylinder so as to be over the flame, and the backward pitch of the furnace wall facing them, or, as the patent expresses it:

"The front and back walls are vertical, as shown at d, d, to a point above the level of the fire door, from which point they incline toward the top cylinders at about the same angles as that at which the cylinders are set I thus obtain room for a large furnace which is so inclined as to force all the flames toward the boiler tubes, and also bring the tubes, to a certain extent, over the fire."

The steam spaces, but not the water spaces, of the upper drums are connected, and there are imbedded side pipes at each end “to provide for the downflow of the water.” It would also appear that, although there was a connection between the mud drums, the main circulation was not crosswise through it, but that there were two rectangular main circulations endwise through each bank of tubes to their respective connecting upper drums down the side pipes to the mud drums, and through them to the water tubes again. In the inclination of the furnace wall we have a step forward towards the perfected function of the retarding brick arch over the fire chamber in Stirling's patent, which, however, in 1888, was shown by Hanrez in the arch V of his patent No. 384,972.

This brief review of the art, which by no means embraces all the patents pertinent thereto, and the satisfactory character of the results attained in the same general lines which Stirling followed some years later, show the field was so fully occupied that the advance made by him was the gradual step of the improver, not the stride of the pioneer. Singly considered, the elements of

his combination were old. He did not discover the principle of circulation, nor was he the first to devise means to effect it; circulation through a mud drum was not original with him; bafflers and means for effectually distributing the heat to the water tubes were known before; compactness of structure and facility of access for cleaning had been attained, and the deposit of scale secured. That he united all these desirable points in a structure combining simplicity, economy, and effectiveness is true; that his combination showed novelty and patentability is, for present purposes, assumed; and to the extent of his specified combinations, and to others using substantially the same elements, or their equivalents, to accomplish the same result in substantially the same way, his rights will be enforced. Further than this we cannot go, nor are his claims entitled to a broader construction.

Construing the claims thus, we turn to the question whether infringement is shown? It is needless to consider respondents' first form of boiler. It was never manufactured or sold, and all purpose to do so was long since abandoned. In the second form there are three connected mud drums, from which three banks of water tubes extend to three upper cylinders, the steam, but not the water, spaces of which are connected. From the water space of the front

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upper drum a bank of tubes, designated in the proofs as “tubes 10," pass back of the baffler wall, and enter the second mud drum. An additional baffler wall is placed back of the middle bank, by means of which the flames are carried down and go in a third pass through and along the water tubes connecting the rear drums. Of this construction complainant's principal expert says: "In the defendants' boiler shown we find in the three connected mud drums an equivalent of the single mud drum shown in the Stirling patent.” If this be true, then, to our mind, the most vital and marked feature of the Stirling device is gone. We lose at once the compactness of structure, the economy of construction, the small number of manholes, and the triangular circulation, which differentiated it from prior structures, and constituted the grounds of alleged novelty. If the single drum is the same as three connected mud drums, then, manifestly, we sap away the life of the Stirling patent, for how could it have issued in view of the prior art, if this be true? But the three mud drums are not the same as one. Conceding the rear drum performs no necessarily individual functional part, that it could be dispensed with, and the tubes from the rearmost upper drums carried into the middle mud drum, the fact still remains, the functional operations of the two remaining mud drums are not the same. The expert for complainant admits, if tubes 10 were introduced into the front drum instead of the second drum, it would make a substantial difference in principle. If this be so, and if, consequently, each of these mud drums exerts a separate, distinct, and individual function, owing to their separation and relative relation to other parts, how can it be said that their separation is a matter of indifference, and that the sum of their individual and separate functions is the function of the single mud drum of the Stirling patent? If infringement occurs when tubes 10 enter drum 5, and noninfringement when they enter drum 4, what follows when the drums are merged in the single drum of the Stirling construction, and tubes 10 enter it? Does the substituted single drum take upon itself the infringing, character of drum 5 or the noninfringing of drum 4? But, apart from all theory and speculation in this regard, the all-sufficient answer is that complainant limited his claims by the element of a single mud drum, and the claim means just what it says. “The claim is a statutory requirement, prescribed for the very purpose of making the patentee define precisely what his invention is; and it is unjust to the public, as well as an evasion of the law, to construe it in a manner different from the plain import of its terms." White v. Dunbar, 119 U. S. 47, 7 Sup. Ct. 72; Stutz v. Robson, 54 Fed. 506; and Keystone Bridge Co. v. Phænix Iron Co., 95 U. S. 274. So, also, we do not find in respondents' device “the water tube, B, connecting the water spaces of the steam and water drums." Can language be more explicit than these words? Can there be any doubt what the patentee meant, specified, claimed? The words “tube" and "connecting" when applied to two inclosures, imply passage from the one to the other through the medium of such tube. There can be no doubt what the connecting water tubes specified by the patentee

were, and their function is explicitly stated by Prof. Cooley, who says:

"The'circulation of water is as follows: The feed water enters the rear upper drum, and passes thence down through the feed-water tubes to the mud drum. It then passes forward and up through the front bank of tubes to the front steam and water drum, thence back through the connecting water pipes into the feed-water drum, again down into the mud drum; forward and up through the front bank of tubes, and so on coutinuously. * This arrangement of drums, tubes, and connecting pipes appears to be a convenient arrangement, and peculiarly adapted to secure this rapid and complete circulation of water," etc.

In point of fact such a connecting tube in form does not exist in respondents' boiler. Does it in substance? Its equivalent is found by complainant's experts in tubes 10, the mud drum 5, and the front tubes of bank 8, and the triangular circuit found in Stirling's device is claimed to exist in respondents' structure from the front mud drum up tubes 7 to front drum 1; thence down tubes 10 to mud drum 5; thence up the front tubes of bank 8 to steam and water drum 2; thence down the rear tubes of bank 8 to steam drum 5; thence through nipple 12 to mud drum 4, the starting point; and thence forward in circuit.

Conceding, what we are by no means prepared to concede under the prior art, and in the absence of all mention of a triangular eirculation in Stirling's patent, that the double quadrilateral circulation as alleged above could be held, in substance, the equivalent of the triangular circulation of the Stirling structure, does such circulation exist? In this connection we cannot too strongly emphasize the law that the burden of affirmatively establishing the fact of its actual existence-not of its mere possibility or probabilityrests on the complainant. Does a fair preponderance of the proofs affirmatively show this? Without at length reciting or analyzing the testimony on that point, or without discussing the experiments and tests which are alleged to be confirmatory of the existence of such a circulation, all of which we have patiently considered, we are satisfied that the complainant has not met the burden of proof imposed upon it by the law, and has not shown by a preponderance of proof that the circulation alleged by it does take place in respondents' boiler. Indeed, the weight of the testimony in this regard is, in our judgment, with respondents. Such being the case, infringement of the first claim of the patent has not been shown, nor, in our view, has infringement of the second claim been established. In it we find “the single mud drum, A,” and “the two elevated steam and water drums, A, A’, having their steam and water spaces respectively placed in communication." This latter element, under the prior art and the specification, we must construe as meaning the connecting steam and water tubes of the prior claim. Thus construing the claim, infringement has not been shown.

The same conclusion must follow with the second patent. It is expressly stated to embody "certain improvements in that class of steam boilers, which I have described in letters patent No. 407,260.” To the construction therein shown there is added a third upper drum, A”, used as a feed-water heater, connected to the next

forward steam and water drum by a steam pipe, bs, and to the single mud drum, A, by water tubes, B', along which the gases are made to travel by an additional baffler placed back of the tubes, connecting the mud drum, A, and the upper drum, A? It specifically refers to the steam and water drums as connected "with each other by steam pipes, b?, and water pipes, b?.” There is no mention of a triangular circulation, and the only reference to circulation is what would possibly be purely local ones, respectively, in the tubes connecting the mud drums with the two forward steam and water drums, viz. the water "enters the mud drum, A, in a heated state, and from the drum the water rises through the tubes, B', B2, into the drums, A? and A?, and it reaches these drums comparatively free from mud.” In this patent two claims were allowed, viz.:

“(1) A water-tube boiler consisting of a single mud drum, A, two elevated steam and water drums, Ai A2, having their steam and water spaces, respectively, placed in communication; the water tubes, Bi B2, extending from the mud drum to the drums, A1 A2; the feed drum, A3; the water tubes, B3. extending from the mud drums to the feed drums; and the pipes b3, connecting the feed drum with one of the steam and water drums,-substantially as de scribed."

"(2) A water-tube. boiler consisting of a furnace structure, a single mud drum, A; the elevated steam and water drums, A1 A2, having their steam and water spaces respectively placed in communication; two sets of water tubes, Bi B2, directly connected at their upper ends with the steam and water drums, and both sets connected at their lower end with the vingle mud drum; the feed drum, A3; the water tubes, B3, connecting the feed drum with the mud drum; the brick arch, D, extending over the fire place, from the wall of the furnace structure, close to the front set of water tubes, Bi; the firebrick partition, C, inclined between the two sets of water tubes, Bi B2; and the fire-brick partition E, situated between the water tubes, B2 and B3,-substantially as described.”

The additions thus made to the former device were not in them. selves novel. Rankine, as we have already seen, had taught that in boiler construction the motion of the steam and water should, on the whole, be opposite to that of the flame and hot gases of the furnace, and had advocated placing feed-water heaters in the line of gas leaving the boiler. The French patent of Grenier, No. 153,938 (1883), went a step further, and showed in a water-tube boiler a progressive circulation of water forward from the rear, and a cir. culation of the gases in the opposite direction, and the feed-water section made an integral part of the boiler itself.

In view of what has existed in the art, in which must be included the first patent to Stirling himself (James v. Campbell, 104 U. S. 382; McCreary v. Canal Co., 141 U. S. 459, 12 Sup. Ct. 40), the only novelty shown was in the combination claimed. As we read the claims, “the single mud drum, A," is an express element, and "the elevated steam and water drums, A', A’, having their steam and water spaces respectively placed in communication," must be construed as noted in disposing of the second claim of the first patent.

The views expressed heretofore render needless a discussion of respondents' third form of boiler. What has been already said applies to it, with the additional fact that the rearmost of its three

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