5G4 The Architectural Review and American Builders' Journal. [Mar., above or below the footing-course of the foundations. And it constantly hap- pens, that certain soft spots will lie in the way, over which Concrete, unless very thick, and consequently very ex- pensive, has no control. Such a spot ma}- occupy the site of an opening, and should be supplied with an inverted arch, which will have two piers resting on its haunches, and thus bind them to- gether on one level, preventing the un- equal sinking, so generally fatal to good construction. But, in case the soft spot should occur under a pier, then it will be necessary to build piers on the firm ground and turn an arch over the spot, on the top of which the required pier can rest. We here speak in the singu- lar number, but it will be understood that many such, if needed, may be used, always providing inverted arches under the apertures, for the reason before given. The parabolic form is the best for such arches. It is the surest for resist- ing thrust ; and, besides this, it has the advantage of not having to be sunk in the ground so deep as the semi-circular arch. The execution of these inverted arches should be very perfect ; as, any settle- ment in them has a bad erfect on the piers, depending upon them, and con- sequently gives opportunity for that fracture, which their presence was in- tended to obviate. Inverted arches may be constructed with facility by moulding their backs in the ground to be occupied by them, and this may be very exactly done, by pressing down an inverted centering, removing it, and smoothing the bed. The setting of the brick or stone then becomes an easy matter. Besides in foundations of buildings, inverted arches are constantly used in constructing sewers. Fig. I. Shows the inverted arches un- der apertures. FIG!. u^v- - - ", ■ Fis. IT. Shows a relieving arch over soft spot {(J). THE VENTILATION AND WARMING OF THE MUNICIPAL HALL, PITTSBURGH, PA. By Lewis TV. Leeds.* rriETAT there should be some artificial -i- force, or power, used to produce the proper movement of the air, in any occupied building of considerable size, is now universally admitted. But, as to the best manner of apply- ing that force, opinions differ greatly. Many engineers think, that there is nothing equal to a fan, for forcing in the fresh air, and, in addition to that, some use a fan, for drawing the foul air out. Others prefer to depend upon the movement caused simply by a variation of temperature ; and consider, that the same amount of heat, required to run a steam-engine, to drive a fan, if applied directly to a well-constructed shaft, will
- Engineer of Veutilatiou aaid Heating, No. 110 Broadway, New York.
