422 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



point when it was not allowed to escape, the upper part of a room always 

 being most impure. 



He next exhibited a model of Mr. McKinnell's patent double current ven- 

 tilator, in operation. The ventilator consists of two tubes, one within the 

 other, leaving a space between them; the inner tube is the longer, and 

 projects above the outer tube at its upper end; both tubes terminate at 

 their lower ends on a level with the ceiling, but fitted inside the bottom of the 

 inner tube is a short sliding piece, to the lower edge of which is attached 

 a circular flange, which projects outward parallel with the ceiling, and con- 

 ceals the opening of the outer tube when viewed from below; both tubes 

 are properly protected on the outside by cowls. The action of the ventila- 

 tor is as follows: The greater length of the inner tube determines the up- 

 ward current to take place in it; it therefore becomes the foul air shaft; 

 the outer tube becomes the fresh air inlet, and the descending current 

 striking against the flange is thrown out on the plane of the ceiling, and 

 so difi'used; by the use of valves the action of the ventilator can be in- 

 creased or diminished at pleasure. 



The ventilator was fitted into the hole in the top of the glass shade, 

 which was used to represent a room; a piece of thick blotting paper was 

 lighted and then the blaze blown out, and the smoking paper held outside 

 the fresh air entrances; the descending current of fresh air took in with 

 it the smoke, which showed the course of the air as it passed inward and 

 over the flange, which threw it out equally in every direction, passing 

 down by the sides of the shade and finally passing upwards in the center, 

 making its escape up the outlet pipe as foul air; a candle was kept lit 

 under the shade, which burnt brightly, but without any agitation such as 

 would have indicated- an inconveniently strong draught. 



A modification of the same plan of ventilation was next exhibited, which 

 showed how the lower rooms of houses might be ventilated without the 

 rooms above them being interfered with; the fresh air is brought in from 

 the side of the house by pipes laid horizontally betwixt the beams in the 

 floor above the room ventilated, and is difi'used from the ceiling in the 

 center of the apartment; the foul air is carried along horizontally in the 

 same way, to the outside wall, and the pipe is then carried up vertically in 

 the wall for a few feet, or in some cases to the top of the house, and the 

 air then allowed to escape. 



Mr. Hyslop was listened to with great interest, and some of his experi- 

 ments elicited hearty applause. 



Mr. Fisher then explained, by means of diagrams upon the blackboard, 

 the patented balance piston valve of T. S. Davis, as applied to locomotives. 



Unlike ordinary pistons, the head and follower are of considerably less 

 diameter than the ring, which is counterbored to receive them. The width 

 of the ring is equal to the entire thickness or depth of the piston, its edges 

 being flush with the outer faces of the head and follower. The latter are held 

 together, and held tightly upon the counterbored shoulders in the packing 

 ring, by being placed on the valve stem, with a nut on each side of the 

 piston. The head and follower are tightened against the inner shoulders 

 of the ring, and not upon each other, for the head does not extend so far 

 through the piston as to allow the follower to take its bearing upon it. 



