492

Position of VacMum Tubes vnthin blowing ones. [Book V.

to indicate that the length and taper of the additional tubes should varywith tbe force of the steam, and that the annular passage for the vaporshould be contracted as the elastic force of the steam was increased.

Cylindrical pipes applied to the mouth of A, or to those of the addi-tional tubes, caused the mercury to fall; but any plane object held againstthe current issuing from A, did not affect the vacuum. A piece of boardwas gradually brought to within one-fourth qf an inch of the end of A, andof course deflected the steam at right angles; yet the vacuum was not inthe least diminished until the board was pushed still nearer. By applyingthe large ends of the additional tubes to A the vacuum was diminished.

The noise made by the steam issuing from A is indicative of the stateof the vacuum. If it be loud and sonorous, the vacuum is not near so highas when the sound is less and hissing. In the former case there is generallytoo much steam escaping—the cock should be partially closed.

No. 218 is a vertical and a horizontal section of the device by means ofwhich the vacuum tube was retained in a position either eccentric or con-centric with the blowing one, and by which it could be drawn to one sideso as to touch the narrowest part of tbe latter. Three fine screws withblunt ends were tapped at equal distances from each other into solid pro-jections cast on A, lj inches below the contracted part. By these, theexact position of the vacuum tube which gave the best result was accu-rately ascertained; and it was remarkable how small a change in its posi-tion affected the mercurial column. A few examples are annexed :—1.The steam in the boiler being low, and tbe mercury Standing at 3J inchesonly, the vacuum tube was drawn by the screws so as barely tö touch A,and instantly the mercury feil to 2 inches. 2. When the pipes were clearof each, and the mercury 19J inches high, as soon as they touched it feilto 16 inches. Simiiar results took place whatever might be the force ofthe steam. 3. The mercury feil also when the axis of both tubes did notquite coincide, although a clear passage still remained for the steam, asshown at E, No. 216. In this case, as in the others, the greater flow ofvapor on one side probably created cross currents in A, after passing thecontracted part. On one occasion, the mercury suddenly feil several incheswhile the pipes were concentric with each other. Upon examination thiswas found to be owing to a minute piece of grit, or a film of lead, blownby the steam between the two pipes, where it was wedged in. It did notexceed one-sixteenth of an inch in any direction. It produced the sameeffect as when the pipes touched. Upon removing it, the steam rose asbefore.

Another point necessary to be attended to is the position of the orificeof the vacuum pipe with respect to the narrowest part of the blowing one;i. e. whether in a line with it, or in advance, or behind it, as figured at AB C No. 219. To test the effect of these various positions, the vacuumpipe was so arranged by a screw cut on it, as to be pushed in or drawnback at pleasure. In one experiment, the mercury stood at 21 incheswhen they were on a line, as at A. The vacuum tube was pushed for-ward three-sixteenths of an inch without any change in the vacuum; butwhen the pipe protruded three-eighths, as at B, the mercury feil to 18inches. It was then drawn behind the contracted part of A, and the mer-cury instantly began to fall. When the orifice was one-fourth of an inchbehind, the mercury feil from 21 inches to 4; and when drawn back one-eighth of an inch more, as at C, the steam descended the vacuum tube andblew the mercury out of the vessel that contained it. In another experi-ment, the vacuum tube was one-fourth of an inch in advance of the con-tracted part, and the mercury 20 inches high : when the tube was drawn