Double Blowing Tube.

493

Chap. 3.]

back so that its end was in a line with the contracted part, the mercuryrose half an inch. When drawn back one-eighth of an inch, it feil toYl\ inches.

That part of the vacuum tube within the steam chamber, or back endof A, should be straight, and its axis should coincide as nearly as possiblewith that of A, eise the vapor in passing over the orifice will be more orless deflected to one side, and thus dirninish the vacuum.

Although the blowing tube figured at No. 217 has its mouth openingupwards, in practice we used it in a horizontal position, as at No. 219, orrather inclined downwards, as at No. 221, that the Condensed vapor mightnot fall back and enter the vacuum tube.

No. 220 represents another modification of this mode of removing at-mospheric pressure, by which the vacuum may be carried toa greater extent than with No. 217. It consists of two blow-ing tubes attached to one vacuum pipe. The lower blowingtube in its narrowest part was seven-twentieths of an inch indiameter, and the annular passage for the steam between itand the vacuum pipe was only one-fiftieth of an inch in width.The bore of the steam cock and pipe C was three-tenths of aninch. The upper end of the lower blowing tube was half aninch in diameter, and terminated at the contracted part of alarger one, D, where a space of one-thirtieth of an inch wasleft for the steam between them. D was six inches long, andits upper end an inch in diameter. It was also furnished withsteam by the pipe and cock E. (Both blowing tubes inthe accompanying illustration are figured too large for theexhausting one.)

When this device was tried, the safety-valve of the boilerindicated a pressure of 40 pounds on the inch. The cock Ewas first opened, and as the steam rushed from D the mercuryrose 8 inches. E was then closed and C opened, upon whichthe mercury rose 8.8 inches. Both cocks were then opened,and the mercury rose 16.6 inches.

When the steam was at 60 pounds and E opened, the mercurial columnwas 9 inches. With E closed and C opened it rose 15 inches. Bothcocks were next opened, and the height was increased to 20 inches.

Steam at 80 pounds and E open, the mercury stood at 17.5 inches. Copen and E shut, it rose to 21 inches; and when both were opened, itreached to 24J inches. The addition of another blowing tube over Dwould most likely have carried it to the full height of the barometer. IfD were inserted in a chimney in the direction of the flue, it would notonly increase the draft, but the draft would increase the vacuum.

The steam pipe that supplied D was then unscrewed from the cock E,which was left open. The cock C was again opened, and the mercuryrose as before to 21 inches, the air rushing through E producing no effecton the column except rendering its surface slightly concave. By oftenclosing and opening the orifice of E with the finger, no sensible change inthe vacuum could be perceived.

After removing the vessel of mercury from the bottom of the vacuumpipe, a piece of twine several yards in length which happened to be layingon the ground near by, was drawn into the tube and discharged throughD. This was repeated several times. By presenting one end near theend of the glass tube, the whole was almost instantaneously drawn up andthrown out by the steam, although the vacuum tube was continued in ahorizontal position nearly two feet before it was connected to the glass

No. 220.