1876.] 



Sprengel Air-pump and Vacuum-tap. 



401 



that may reach here before entering the apparatus attached to the 

 pump. 



I will now refer to some of the mechanical arrangements fisLed to the 

 stand for the convenient working of the instrument. 



The reservoir A is fixed on a slide running up and down the stand 

 in grooves cut in separate pieces of wood and screwed to the stand. The 

 stops S' and S" are of brass, and kept jntting out from the stand by a 

 spring from behind ; they are pressed in when the slide passes over them, 

 and fly out again directly underneath it. When lowering the reservoir, 

 the stops are pressed back by the thumb. 



The pinch -cock K is especially contrived for rapid working. It is 

 represented in fig. 6. The lever {a) is made of oak, one end being loosely 

 screwed to the stand with an ordinary screw ; the other end is cut into a 

 fork to receive the fine threaded screw (5), which carries two nuts, and 

 moves backwards and forwards on the pinned joint (c). Xow when once 

 the nuts on the screw (6) have been screwed down on the fork of the 

 lever, so as to siifiiciently compress the pipe, it is then entirely opened or 

 shut by simply working the screw on the joint (c) : the open position is 

 shown by dotted lines. When closing this pinch-cock, the lever is pressed 

 down by the thumb while the first finger places the nuts over the fork. 



The pinch-cocks X, M are made in the same way as the above, but 

 without the joint c, the gradual opening and shutting by means of a 

 screw being necessary. 



The stand is 5 feet 6 inches high and 11 inches broad. The fall-tubes 

 measure about 36 inches from the highest level of mercury in B to the 

 jet : the two side ones ha\ e a bore of about 2 millims. ; the centre one 

 is smaller (about I5 to 1| millim.), being the size according to my 

 experience to produce the most complete vacuum. The auxiliary side 

 tubes are made of the larger bore for the sake of rapid exhaustion up to the 

 point at which the barometer and gauge appear to be level, when ti e 

 centre tube does the principal work, although the side ones still aid con- 

 siderably. 



The reservoir A contains, when full, rather more than 20 lbs. of mer- 

 cury, being a convenient weight to lift. With regard to the capabilities 

 and advantages of this pump it is unnecessary for me to dwell at any 

 length, as the \ acuum that the " Sprengel " principle is capable of pro- 

 ducing is so well known at the present time ; although I may safely say 

 that the exhaustion produced by this instrument will at least rival, if not 

 considerably exceed, that produced by any other air-pump yet brought to 

 bear upon research in vacua. The rapidity of working is of course the 

 chief advantage of an instrument of this construction. 



Experiment has shown that the raising of the reservoir and passing of 

 its contents through the pump five times will reduce the pressure within an 

 apparatus having a cubic content of about 80 cubic centims. to '041 

 millims., measured by McLeod's apparatus. But with regard to these 



