Before it is brought to the test let us explore that shell. To our eye, its external appearance is more pleasing than that of the building we just left. The one central and four terminal towers, with their open, kiosk-like tops, are really graceful, and the slender spires which surmount them are preferable to the sham of sheet-iron turrets. Thanks, too, to the necessity of projecting an annex for hydraulic engines from one side of the middle, the building is distinguished by the possession of a front. The main cornice is forty feet in height upon the outside; the interior height being seventy feet in the two main longitudinal avenues and forty feet in the one central and two side aisles. The avenues are each ninety feet in width, and the aisles sixty, with a space of fifteen feet for free passage in the former and ten in the latter. A transept ninety feet broad crosses the main building into that for hydraulics, bringing up against a tank sixty by one hundred and sixty feet, whereinto the water-works are to precipitate, Versailles fashion, a cataract thirty-five feet high by-forty wide. The substitution of timber for iron demands a closer placing of the pillars. They are consequently but sixteen feet apart "in the row," the spans being correspondingly more contracted. This has the compensating advantage, æsthetically speaking, of offering more surface for decorative effect, and the opportunity has been fairly availed of. The coloring of the roof, tie-rods and piers expands over the turmoil below the cooling calm of blue and silver. To this the eye, distracted with the dance of bobbins and the whirl of shafts, can turn for relief, even as Tubal Cain, pausing to wipe his brow, lifted his wearied gaze to the welkin. Machinery Hall has illustrated, from its earliest days, the process of development by gemmation. Southward, toward the sun, it has shot forth several lusty sprouts. The hydraulic avenue which we have mentioned covers an acre, being two hundred and eight by two hundred and ten feet. Cheek by jowl with water is its neighbor fire, safe behind bars in the boiler-house of the big engine; and next branches out, over another acre and more, or forty-eight thousand square feet, the domain of shoes and leather under a roof of its own.