City IjtPBOVsitaKra.—The Board of City Improvements held a special session yesterday morning, at which Mr. Gilbert, City Civil Engineer, presented plana and specifications for erecting the new engine house, upon the we»tlot of the Mechanics’ Institute. It is proposed to be, from bottom to extreme top about 200 feet high ; of rough free atone work in front. There will be four stories, two for public halls and two for offlcea. It is thought to be a good plan, as there will be four stories in front and seven in the rear, which will be much higher than the Mechanics’ Institute. The lot is fifty feet wide by fifty fe**t deep. The building, the style of which is to be Gothic, will oocupy the whole lot. Underneath the whole will be a cellar nine feet deep; as table in the rear, divided into seven stalls, with a window over each.Without considering ourself a very competent judge, we think the design does credit to Mr. Gilbert.