V. MASS. 75 



air-tight at a height of about + 600 mm. Then funnel 2 is sunk to about 

 1,000 mm. below the zero point of the reservoir. The mercury level will 

 thereupon sink below the reservoir. In case reservoir and hose 1 were 

 already exhausted of air, the difference of the level of both the mercury 

 menisci will be exactly as much as indicated by the barometer, otherwise the 

 difference will be smaller. All the mercury then flows back from the reser- 

 voir, &c. into the funnel 2, for which reason the same must have a sufficiently- 

 large space of capacity, and must be sunk carefully, not too quickly. Cock 

 No. 1 is then shut, funnel No. 2 lifted above the zero point of the reservoir, 

 and the latter, which in the most unfavourable case will contain only extremely 

 rarified air, filled by the same ; then cock 1 is opened, and the cramp at the 

 hose No. 1, so that the mercury in the funnel No. 1 can rise again. ^This 

 exhausting of the air is repeated a second time if necessary. 



For testing whether the reservoir is entirely or sufficiently exhausted of air, 

 funnel 2 is sunk so far until its mercury level has reached about 400 mm. 

 below the zero point of the reservoir. Now occurs a Toricelli's vacuum in 

 hose 1 , and the mercury meniscus is seen in the glass tube. 



If there were still air in hose 1, the level-difference in the hoses 1 and 2 will 

 be smaller than the height indicated by the barometer for the day in milli- 

 meters. In this case the operation mentioned before is repeated. In all cases 

 will, at the utmost at a position again of funnel 2 at about 440 mm., the 

 difference of the level of both the mercury menisci be smaller by 2 3 mm. 

 than the indication of the barometer for the day. If the difference in the 

 variation of .the levels should show itself equal to the height of the barometer, 

 what may be easily ascertained by the scale, if its at the lower end pointed 

 steel sole touches the mercury in funnel 2, by adjusting the slider fastened at 

 the height indicated by barometer at the upper meniscus in the glass tube ; 

 the pressure at the upper part of the reservoir will then be 



1 atmosph. air pressure 1 o mm 



1 atmosph. mercury pressure J ~ 



consequently the reservoir exhausted of air. 



But in order to employ a further powerfully-acting means for exhausting 

 the air, so far as this should not have been accomplished already, the 

 funnel 2 is lifted as high as possible up to about 2, 760= 1,900 mm., thereby 

 the very small quantity of air still present will be forced into the hose 1 

 under 2^ + 1=3| atmospheric pressure, and will ascend either towards the 

 hose 1, or occupy only a small and practically insignificant place, of 0-001 to 

 O'OOOl cub. cent, by shutting off the cocks 1 and 2. 



The raising and lowering of hose 1 and 2 is performed by pulling or 

 slackening of the cords running over the rollers fastened into the ceiling of 

 the room. 



It may now be supposed that the air is completely exhausted from the 

 reservoir, and that its vacuum is completely filled with mercury. At all 

 events the hydrostatic air pump of to 3'5 atmosph. pressure, attached to the 

 apparatus in the simplest manner possible, will act much more powerfully 

 than any other air pump, in which the so-called injurious space in the venti- 

 lator of the piston makes it impossible to increase the suction action to 

 O atmosph. pressure. 



Finally, the two cocks 1 and 2 in the mercury are shut off, the temperature 

 of the latter being determined, the two hoses 1 and 2, which have previously 

 been carefully emptied, are unscrewed, the reservoir cleaned of the mercury 

 globules sticking to it (especially in the parts of the screw and the interior 

 channel-openings of the cocks), and the weight of the reservoir, including 

 bolt, determined with mercury. 



