BETWEEN 75 AND 150 MILLIMETRES OF MERCURY. 
419 
reservoir E, to which, as to the manometers, mercury can be supplied from below. 
The lower chamber C of the right-hand manometer is connected with the principal 
gas reservoir. This consists of two bulbs, each of about 129 cub, centims. cajiacity, 
connected together by a neck of very narrow bore. Three marks are provided, 
one G above the upper bulb, a second H on the neck, and a third I below the lower 
bulb, so adjusted that the included volumes are nearly equal. The use of the 
side-tube JK will be explained presently. 
When, as shown, the mercury stands at the lower mark, the double volume is in 
action and the pressure is such as will balance the mercury in one (the right-hand) 
manometer, A vacuum is established in the upper chamber Jd from which a way is 
3 H 2 
