Our architects having increased with the demand for finer houses and more showy public buildings, and having parted company with their Greek and Roman idols to which their predecessors had been so long and so faithfully wedded, and acknowledging the necessity for novelty, ardently embraced the newly arising fashion and the Mansard roof arose at every corner in all its glory. At first the compositions which were adorned with this crowning were pleasing to the general view, if not altogether amenable to the strict rules of critical taste. But in due time (and alas that time too surely and severely came) the _pseudo_ French style with its perverted Mansard roof palled upon the public taste for the eccentricities its capricious foster-fathers in their innate stultishness compelled it to display. Some put a Mansard roof upon an Italian building, some on a Norman, and many, oh, how many, on a Romanesque! Some put it on one story erections and made it higher than the walls that held it, in the same proportion that a high crowned hat would hold to a dwarf. Some stuck on towers at the corners of their edifices and terminated them with _Mansard domes_! Some had them inclined to one angle, some to another; some curved them inward, some outward, whilst others went the straight ticket. The dormers too came in for a large share of the thickening fancies and assumed every style or no style at all. The chimney shafts were not neglected. Photos of the Thuilleries were freely bought up, and bits and scraps of D’Lorme were hooked in, to make up an original idea worthy of these smoky towers. “Every dog will have his day,” is a fine old sensible remark of some long-headed lover of the canine species, and applies alike to animals, men, and things. That it particularly applies to that much abused thing called the Mansard roof is certain, as the very name is now more appropriately _the absurd roof_. Fashion begins to look coldly upon her recent favorite, which in truth “has been made to play such fantastic tricks before high Heaven, as make the angles weep;” and it is doomed. A few years hence, and we will all look back in amused wonder at the creations of to-day, crowned with the tortured conception of MANSARD. HYDRAULIC CEMENT. The rapid hardening under water of the cement which from that property derives its name of “Hydraulic Cement,” has been, and indeed is still, a subject of discussion as to the true theory of such action. We find in the June number of the _Chemical News_ a paragraph which must prove very interesting to manufacturers as well as to all who use and take an interest in that most useful of building materials to which the Architect and the Engineer are so deeply indebted.