490

Apparat.us for employing Currents of Steam.

[Book V.

an increase of the steam’s elasticity increased the vacuum, an increaseddischarge of the vapor was often found to diminish it. This was frequentlythe case when high steam was employed : for example, if the cock throughwhieh steam passed into the blowing tube marked C in No. 217 waswideopen, the mercury would sometimes fall two or three inches, but whenpartially closed, would instantly rise ; thus indicating that it is the velocityand not the volume of vapor passing over the orifice of the exhaustingpipe, upon which the vacuum depends.

No. 216.

No. 217.

No. 218.

No 219.

We first passed steam through tubes connected like No. 213, both withand without the conical ajutages C D in Nos. 211 and 212. Various pro-portions of the steam passage over the orifices of the vacuum or exhaustingpipes were also employed, as at A, B, C, D, E, No. 216, which repre-sent horizontal sections of the vacuum pipe and steam passage over itsorifice. The dark parts show the passage for the steam, and the innercircle the mouth of the vacuum tube. In A the steam channel did notextend over one-fourth of the circumference of the orifice ; in B it reachednearly half way round; in C three-fourths ; while in D and E it extendedentirely round. Upon trial, the vacuum produced by B was greater thanthat by A; C surpassed B, and D uniformly exceeded them all. Wetherefore finally arranged the apparatus as shown at No. 217, in which Ais a brass tube composed of two conical frustums United at their lesserends. The longer part, A, was smoothly bored and polished in the direc-tion of .its length, to remove any minute ridges left by the borer. Theinterior diameter of the large end was an inch and an eighth, and of thesmallest part nineteen-fortieths, (rather less than half an inch.) The ex-ternal diameter of the vacuum pipe B was seventeen-fortieths, so that theannular space left round it for the steam was only one-fortieth of an inchin width, being about as small a space as could well be formed withoutthe pipe B touching A. The length of A from the contracted part was5J inches. A glass tube three feet long, whose lower end was placed in