II. AIR PUMPS, ETC. 1(33 



the acquired vacuum preserved. To effect this, the stop-cock is furnished 

 with two iron rings, which float on the mercury, and, falling with it, close the 

 opening of the stop-cock. By means of the second perforation of the stop- 

 cock the instrument can he joined to an ordinary air pump, and the operation 

 abridged by a partial exhaustion of the air. To produce a more complete 

 vacuum than can be obtained by an ordinary air pump, the stop-cock is turned 

 half round, and, when the vessel surrounding- the stop-cock is filled with 

 mercury, the Sprengel pump begins to act. 



589. Aspirator, moved by clockwork with sulphuric acid ; 

 U tube for determining accurately the amount of moisture in the 

 atmosphere by the use of the balance. Dr. Andrews, F.R.S. 



The amount of moisture at different periods of the day, or the average 

 amount in 24 hours, may be determined with great precision by means of this 

 arrangement, which the exhibitor proposed many years ago for use in meteor' 

 ological observatories, instead of the present defective methods. 



590. Spirator. Designed to get a constant current and 

 known volume of air driven or drawn over a body. . 



Frederick Guthrie. 



The principle resembles that of " Tantalus' cup." A constant current of 

 water enters a flask, and (1) drives out its own volume of air through a 

 mercury trap ; (2) when the flask is filled up to a certain point, a siphon acts, 

 and, in emptying the flask, draws air in from another tube. 



590a. Apparatus for collecting Gases without exercising 

 upon them either pressure or rarefaction. George Gore, F.R.S. 



612. Apparatus for Air Pump. An air-gun supported 

 by two vertical mahogany pillars and cross-bar, by means of which 

 it may be adjusted to any angle. 



The Council of King's College, London. 



613. Apparatus for Air Pump. Metal condenser with 

 glass ends, large enough to take a pair of four-inch Magdeburg 

 hemispheres with fittings, and a metal lever to which weights may 

 be attached to measure the pressure of the air, either when com- 

 pressed or at its ordinary pressure. 



The Council of King's College, London. 



592. Drawing of Mercurial Air Pump (1872). 



J. P. Joule, D.C.L., F.R.S. 



By alternately lifting and. lowering the bulb attached to the flexible tube, 

 the air being dried by the admission of sulphuric acid through a glass valve 

 at the upper part of the perpendicular tube, a very excellent vacuum may be 

 obtained in a short time. 



592a. Sprengel's Mercurial Air Pump, improved form. 



E. Cetti $ Co. 



592b. Modification of Sprengel's Air Pump, with air- 

 trap and means of supplying sulphuric acid, in order to clean out 

 the fall tube while the pump is in use. Prof. H. McLeod. 



L 2 



