tory work on account of the slowness of its action. The drawback is oxercome, to a great extent, by supplying the pump with a number of fall-tubes, which act together as a single one. For example, if six fall-tubes are used, the work of removing the most of the air is done in one- sixth of the time required by a single jiumji. After the greater part of the air is removed, how- ever, the time taken to produce a good vacuum is not nearly so much reduced, and it is chiefly in the early part of the operation where the saving of time is effected. Another drawback to all mercury pumps is their liaViility to break- age, even with the most careful usage. In the Sprengel pump, owing to the continual hammer- ing of the mercury, the fall-tubes are very often broken, even after only a very short usage. A method is in use with both of these forms of pumps which consists of exhausting into a par- tial vacuum instead of into the atmosphere. This is accomplished by inclosing the part of the apparatus where the air is expelled in a cham- ber which is kept at a partial vacuum by means of a meclianical or water air pump. By this means the mercury pump will work against a pressure much less than the atmospheric pres- sure, and consequently the fall-tubes and the height to which the mercury must be raised can be very much reduced, while the air is much more readily drawn down and out of the fall- tubes. In factory work the raising of the mer- cury from the lower to the upper level of the piniips is done mechanically and not by hand. It may be raised by a force-pump, or in small buckets on an endless chain, or by air pressure. The latter may be simply atmospheric pressure, and the mercury is raised by being broken up into small lengths with air spaces between, like a Sprengel pump working upward into a vacuum chamber. The illustrations show modern forms of mercury pumps. In an improved form of Sprengel pump designed by G. W. A. Kahlbaum a steel gun-barrel replaces the glass fall-tube. This avoids the electrification of the glass by the friction of the falling mercury, and with the other improvements introduced enables a higher vacuum to be attained than ever previously. In this way, in 1901, he was able to obtain a degree of exhaustion corresponding to a pressure of .0000018 millimeters of mercury, which is considered the best on record.
The degrees of exhaustion reached by the various mercurial air pumps may be seen from the following table adapted from Müller-Pouillet, Lehrbuch der Physik:
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Geissi^er's Pump. (Older t.vpp.) KBAVoWy's Pnwp. Geisbi.er's Pump. (Later t.vpe.) Beesel-Hagen. V. WaltenhofeQ. Bessel-Hagen. C C X c c t 5.= c ^ c c E <s,'Z a e
o III
£ 1 =
Amount 0( exhaustion, in fractions of an Crookes. ,000046
HAM PtrMl ,000,000 Bessel-Hagen. .000009
(Later type.) ,000,000 Sprencel - GiMise- HAM Pump. ("W'itli improve- Eood. .0000069 .000008
,000,01)0
ments.) ,000,000 Sprengel Pump. (With improve nieiUs, 1901.) Kahlbaum. .0000018
420,000,000
- e8S
The water pump invented by Bunsen is a simple form of apparatus that is found generally in physical and chemical laboratories, and adequately answers when too high a degree of exhaustion is not required. It consists of a tube attached to a faucet or other supply of water under pressure, through which water empties into a chamber provided with two outlets. From one of these the water flows out, carrying with it the air from the vessel to be exhausted, which is connected with the second tube. In its original form this piece of apparatus was made of glass and rubber tube, but with metallic parts that allow it to be connected to an ordinary faucet. It is extensively used in laboratories. The page illustration (Fig. 3) shows one form of such a water pump. Descriptions of air pumps of various forms are to be found in all the large treatises on physics, including those of Ganot, Deschanel, and Müller- Pouillet (Brunswick, 1886), the latter (in German) giving a complete account of the most important types of apparatus of this class. In the Journal of the Society of Arts, Volume XXXVI. (London, 1888), there is an interesting and valuable article on "The Development of the Mercurial Air Pump," by S. P. Thompson, in which the various forms of this instrument are described. This has been reprinted in book form. The reader is also referred to the columns of the Annalen der Physik und Chemie and the American Journal of Science, in which are described many forms of air pumps and vacuum apparatus.
AIR RESIST'ANCE of a Projectile. See Ballistics.
AIRY, ar'i. Sir George Biddell (1801-92). An English astronomer. He was born at Alnwick, Northumberland, and graduated at Trinity College, Cambridge, in 1819. In 1825, he discovered the optical defect of astigmatism and provided a corrective for it. He was elected to the Plumian professorship at Cambridge in 1828, and intrusted with the management of the Cambridge Observatory, the results of his labors being published in the compilation entitled Astronomical Observations, 9 volumes (Cambridge. 1829-38), which became the model of all analogous works since published in Great Britain. In 1836 he suc-
