locities of the currents are unequal, the greater velocity diminishes the less, destroys it, or inverts it, according to the excess of velocity. The knowledge of this fact leads at once to the interposition of a plate, to prevent loss of velocity in interfering currents.

"5. A thin plate placed upon the extremity of a tube, at the proper angle, causes the impinging current to assume a certain direction, and to produce a certain velocity in the tube; a similar plate parallel to and above this plate does not increase that velocity.

"A cone placed upon the extremity of a tube produces similar changes of direction in the impinging current, and similar movements in the tube, but another cone above the first does not increase the velocity of those movements.

"6. Beyond certain narrow limits, the velocity produced in a tube by the action of a current on its conical extremity is not increased by increasing the height or diameter of that cone. The full effect of a cone may be obtained when its lower base is not larger than one half, nor less than one third, the diameter of the flue on which it is placed. ". If a flat truncated cone be fitted to the extremity of a tube, and exposed to the impinging current, a velocity may be produced in the tube of 1.71 feet per second; if a similar but much smaller hollow truncated cone be inverted and closely secured to the mouth of the first, the velocity in the same tube may by this means be increased to 2.21 feet per second. The same increase of velocity will be produced if the internal cylindrical bore of the first cone be made conical, with its larger base upward. By the addition of this secondary cone, or by the modification of the interior of the first cone, the velocity of the current is increased over that produced by the simple cone nearly in the ratio of 10 to 13, and as the effect is as the square of the velocity, its efficiency is increased nearly in the ratio of 10 to 17. This is the best form of the simple fixed cone, and the most efficient fixed ventilator, which has been examined by the Committee. Venturi has shown, that, when a conical tube is applied to a cylindrical pipe, the larger base of this conical tube being 1.8 the diameter of the pipe, and its height 9 times the diameter of this same pipe, the expenditure will, with water, be greater for the cone than for the cylindrical pipe, in the proportion of 24 to 12.1.

"8. A hollow truncated cone, with its larger base closed by a flat plate, inverted and placed above a cone similar to that last described, will increase the velocity of the current in the pipe upon which it is

placed over that produced by a simple cone nearly in the ratio of 10 to 13. This is one of the most efficient fixed ventilators with a cap which have been examined by the Committee. The form described in the preceding paragraph, with Cisalpin's plate placed at a certain height above it, is to be ranked in efficiency with that last described.

"9. The velocity of the current produced in a pipe, the mouth of which is presented fairly to the blast, is nearly constant, whether the mouth be cylindrical, conical, with its larger base towards the blast, or the reverse. The diminished area exposed to the blast, in the latter case, is counterbalanced by the increased velocity consequent upon diminished atmospheric pressure within the cone.

"10. A difference of temperature between the impinging blast and the produced current does not, within the limits observed, influence the velocity of the latter.

"EXPERIMENTS.

"In the experiments, each ventilator, when examined, is placed upon a perpendicular fixed tube of tinned iron, in the centre of the mouth of the trunk of the blowing machine. This and all other tubes, when not otherwise mentioned, are 1.25 inches in diameter. The velocity of the blast is 10.36 feet per second, or, as indicated by the revolutions of the handle of the blowing-machine, one revolution per second. The time required for the chlorine to act upon the starch, from the moment it is introduced into the pipe, is given in seconds; the velocity of the current is given in feet and decimals. The direction of the blast is indicated by the

66

Exp. 4. Elbow; horizontal portion 4 inches long, opening turned from the blast,

"Exp. 5. Same; horizontal portion making, with the direction of the blast, an angle of 30°,

"Exp. 6. Elbow turned from the blast, and having around its opening a plane surface 1.5 inches wide,

. 31.0 1.71

"Exp. 7. A perpendicular
plate 2 inches wide and 1.75 inch-
es in height, fastened to that side
of the fixed tube next the blast, . 33.0
"Same plate attached to the

1.61

. 48.0

1.05

fixed tube, but with its edges in the same direction
with the blast,
"Same plate on the side of the fixed tube, opposite
the blast,
no effect in 180.0
"Exp. 8. A square plate, 2 inches by the side, on
the top of the fixed tube on the side next the blast,
"Same plate making with horizon an angle of 80°,

31.5

1.68

28.2

1.87

66

Exp. 9. Square plate, 2 inches by the side, with vertical edges .5 inch wide, turned from the blast, and making an angle of 45° with its direction; the whole plate making an angle of 75° with the horizon,

"Same plate, making same angle with the horizon, but with its edges turned in the opposite direction; that is, towards the blast,

66

Exp. 10. A plate 1.25 inches wide at the base, 2

inches wide at top, and 2 inches high, with its edges turned towards the blast, as in the last experiment, gave very nearly the same results, .

"Exp. 11. A plate 2 inches wide at the base, 1.25 wide at the top, and 1.5 inches high; angles of sides with base equal to inclination of the plate with the horizon, 76°; placed on the top of the fixed tube, on the side next the blast, its base being raised .37 inch above the mouth of the fixed tube,

29.5 1.80

"A similar plate added to the opposite side of the tube,

"Similar plates on three sides; open side from the

blast,

33.5

1.58

"Similar plates on three sides; open side at right angle with direction of the blast,

"Similar plates on four sides,

"Exp. 12. Pyramid formed by the four plates, as last arranged, with its base so fitted to the top of the fixed tube that no air could enter by its side,

"Exp. 13. Two similar plates, those used in the last experiments, one arranged as in Exp. 10, and the other similarly placed, but raised .37 inch above the first,

"The influence of the inclined plate, used in several of the preceding experiments, would at once suggest the application of a figure of revoolution, which would have a similar effect upon the blast, that is, would direct it upward, and thus assist the escape of the current from the tube. A cone is evidently one form which would have this effect. Indeed, the conical chimney-top has been long in use, and its principle often reproduced under slight modifications of form.

"The cone was proposed as a proper form for the chimney-top, and an account of its application published, more than seventy years ago, by Count Cisalpin, in a memoir entitled Description d'une Cheminée et Étuve de Nouvelle Invention. The plan contrived by Cisalpin consisted of truncated cones of plate or sheet iron, of different sizes. When this apparatus is to be used,' says he, ' fit to your chimney your first size; it is of no consequence whether the chimney be round

་

or square, provided it have no holes in its sides, and is open only at the top; if this put a stop to the smoking, your object is probably accomplished, the equilibrium between the wind and smoke is destroyed (nevertheless assure yourself of this by many experiments, made at

Fig. 6.

different times), and then you have nothing further to do than to attach to three sides of the cone three rods of iron, four, five, or six inches long, on which place horizontally a round plate, having a diameter a little larger than that of the cone, to prevent the rain from entering the chimney.' The adjoining figure is an elevation from the perspective view given in the memoir.

"In 1788, De Lyle de Saint-Martin, a lieutenant in the French navy, again called attention to this form of chimney-top, in a memoir, giving a full description, with drawings, of its construction, and the results of his experiments. The cap surmounting the cone, instead of being flat as in Cisalpin's, was also a truncated cone, but differing in its proportions from that forming the chimney-top. arrangement, which is here figured from Saint Martin's memoir, was examined and approved by the French Academy of Sciences, and published in its Transactions.

Fig. 8.

This

"Mr. Tredgold, in his treatise on Warming and Ventilating Buildings, published in 1824, and still a standard work, refers to the conical top as one which may often be employed with advantage, when formed in the manner described in fig. 8; and remarks, The upper cap prevents down blasts of air, but in a steady horizontal wind the lower cone alone would be sufficient.' Its mode of action is described and illustrated by figures, from one of which the annexed cut is copied. For its origin Mr. Tredgold refers to the memoir of De Lyle de Saint-Martin. It will be noticed that the conical cap has, in the last figure, assumed the spherical form.

"The annexed cut shows the same truncated cone, which has, during the past year, been introduced as quite a novelty, the inventor having gone back to first principles, and again mounted the flat top.