169 



AIR-PUMP. 



AIR-PUMP. 



170 



ifln 



the degree of rarefaction. For example, suppose the common barometer 

 to be 30 inches high, while the barometer-gauge of the air-pump 

 .-tands at 20 inches. If the vacuum were complete, the barometer- 

 gaiige would be a common barometer, and would stand at 30 inches : 

 but as it stands only at 20 inches, the pressure of the air in the 

 receiver is equivalent to 10 inches of mercury, or one-third of that of 

 the exterior air. Therefore the density of the air in the receiver is 

 one-third of that of the exterior air, or two-thirds of the air have been 

 removed. 



The preceding cut shows a section of the piston rod, as well as of 

 the barrel. The tube m comes from the receiver, and air can be 

 admitted by it into the barrel, when the rod ytj is raised. The rod ay 

 passes into the piston-rod (which is hollow), and works stiffly in it, 

 being however unconnected with it except by friction. This rod con- 

 sists of two parts, above and below L, the latter of which is not thick 

 enough to till the orifice in which it plays. But when the piston 

 descends, the conical juncture of the thicker and thinner parts is 

 wrought upon this orifice, and shuts it close. After this, and during 

 the rest of the descent, the hollow piston-rod slides downwards upon 

 the rod gy. As soon as the piston begins to ascend, the rod >jij is raised 

 with it, owing to the friction, so far as the nut o will let it rise, after 

 which the piston-rod slides up yrj. We have here the lower valve of the 

 exhausting syringe, shut during the descent 

 of the piston, open during the ascent, and 

 not opened by the force of the air from under- 

 neath, so that the functions of the string 

 which we supposed in our first exemplification 

 are performed. A little higher up the barrel we 

 find the piston, as better shown in the adjoin- 

 ing figure. The external part is a partial pis- 

 ton not connected with the piston-rod, but 

 fitting closely to the barrel. The piston-rod, 

 when rising, fits this exactly, renders it air- 

 tight, and causes it also to rise. But when 

 the piston-rod is descending, it will not cause 

 the descent of the exterior, and, as we have 

 called it, partial, piston, until the project- 

 ing shoulders aa (in the figure) come upon 

 it ; and, as these shoulders do not go all the 

 way round, the piston in descending is not 

 air-tight. This apparatus supplies the place of 

 the upper valve, being air-tight in the ascent, 

 but not so in the descent. Looking above the 

 piston, we hud that its rod works in metal 

 shoulders, the interval between which is occu- 



I pied by stiff leathers. The space above the 



leathers opposite to o is filled with oil, which 

 is communicated slowly to the leathers, and 

 also t<> the barrel beneath. From the latter, 

 however, it is immediately expelled by the rise 

 of the piston, which forces it, as well as the air 

 in the barrel, through the channel aa. The 

 oil and the air then force up the rod in the 

 cavity K, which rod, working in collars, answers 

 the purpose of a valve. The oil is there lodged, 

 until it is collected in sufficient quantity to 



flow again into the reservoir at T. The air escapes into the exterior 

 atmosphere. 



Having shown that we have here an under valve shut during the 

 descent, and open during the ascent, with an upper valve open during 

 the descent and shut during the ascent, we need not repeat the manner 

 in which the rarefaction is produced. We have only further to notice, 

 that a branch from the main tube which enters the receiver is carried 

 through the under wood-work in front, and emerges at B. It is here 

 stopped by a screw ; but when the operator desires to restore the air 

 under the receiver, he opens this screw, upon which the communication 

 between the exterior atmosphere and the receiver is restored, and the 

 air rushes in. In the perspective figure, a cross bar, in which the 

 upper parts of the barrels are inclosed to strengthen them in their 

 position, is omitted for the sake of clearness. 



We give in the following figure a representation of a more portable 

 and less expensive species of air-pump, which, after what we have 

 slid, will need no description. 



The small plate behind the receiver is for another small receiver, 

 in wliich a gauge is placed. This guage is nothing more than a 

 common barometer, which falls with the diminution of pressure 

 from the air in the receiver, in the same way as the common baro- 

 meter when the pressure of the exterior air is lessened by a change of 

 weather. 



T!M' following experiments are among the most common of those 

 shown with the air-pump : 



1 . If the receiver be open at both ends, and the upper orifice be 

 stopped by the hand, on exhaustion the pressure of the exterior air 

 will prevent the removal of the hand. If a piece of bladder be tied 

 tightly over the orifice, as the exhaustion proceeds the bladder will be 

 pressed inwards, and will finally burst with a loud report. The pres- 

 ( the air is also proved Ijy the experiment of the hemispheres, 

 dencribed in the article Am. 



2. The weight of the air is proved by exhausting a copper ball 

 furnished with a stop-cock, which is shut before the ball is removed 

 from the air-pump. It will then be found to weigh less than before 

 the exhaustion was made. 



3. The presence of air in various substances may be detected. A 

 glass of liquid placed under the receiver will give out bubbles of air as 



soon as the exhaustion begins. A shrivelled apple will be restored to 

 apparent freshness by the expansion of the air which it contains, but 

 will resume its original appearance when the air is allowed to return. 



4. The elasticity of air may be shown by placing a bladder under the 

 receiver, not distended, and the mouth of which is tied up. On 

 exhausting the receiver, the air contained in the bladder will expand it 

 more and more, as more of the pressure from the exterior is removed ; 

 and the bladder will finally burst from the interior pressure. If a hole 

 be made in the smaller end of an egg, and the egg be then placed in a 

 wine-glass with this end down under the receiver, the small bubble 

 of air which is always found in the larger end, will, by its expansion, 

 force out the contents of the egg. On re-admitting the air, the con- 

 tents of the egg will be forced back into the shell. 



The first vacuum was made by Torricelli [TOEKICELLI, in Broo. Div. ; 

 BAROMETEB], but the first air-pump was constructed by Otto von 

 Guericke, who exhibited it publicly at the Imperial Diet of Ratisbon in 

 1654. It was an exhausting syringe, attached underneath to a spherical 

 glass receiver, and worked somewhat like a common pump. The 

 syringe was entirely immersed in water to render it air-tight. Shortly 

 afterwards, Boyle constructed an air-pump in which the syringe was 

 so far improved that the water could be dispensed with. He also first 

 applied rack-work to the syringe. The second syringe and the baro- 

 meter-gauge were afterwards added by Hawksbee, and several minor 

 improvements were made by Gravesande and Smeaton. All the 

 alterations which have been made since the time of the invention, how- 

 ever important, relate to the mechanism only, and not to the principle 

 on which the pump acts. 



In the Great Exhibition of 1851, among the instruments exhibited 

 in Class X., the best air-pump was by Mr. Newman. It has a ground- 

 glass plate, and is furnished with two pumps with metal valves, on one 

 of which are two barrels, open at the top. By this arrangement the 

 receiver may be quickly exhausted to 0'4 inch or 0'5 inch. The other 

 pump has a single barrel, with an oil-cistern at the upper part, and the 

 air is lifted through a valve at the bottom of this cistern. " If any- 

 thing re-enters the ban-el it can only be oil, which is brought out with 

 the air at the next up-stroke of the piston. The piston has a metal 

 valve; but the opening of this valve is not necessary to the continuation 

 of the exhaustion, as the piston at its lowest point passes below the 

 aperture leading to the receiver. This construction of air-pump ex- 

 hausts more thoroughly than any yet known." (' Jury Report.') In 

 some experiments which were tried with this pump, the reading of the 

 barometer at the time was 30'08 inches, while the gauge of the pump 

 stood at 30-06 inches. 



A pump exhibited by Messrs. Watkins and Hill, on a plan suggested 

 by Mr. Grove, has oil-silk valves, and is so constructed as to leave the 

 least possible residue of air in the valve after each stroke of the piston. 

 " The piston is solid, without a valve, and the shape of its lower part 

 is an obtuse cone. Part of this cone rises at the top of each stroke 

 above the aperture leading to the receiver; and the air which has 

 entered the barrel is, by the down-stroke, forced through a valve at 

 the apex of the hollow cone terminating the lower end of the barrel, to 

 which the lower end of the piston fits very accurately. The piston- 

 rods pass through air-tight leather collars in the tops of the barrels. 

 This pump exhausted the air till the elastic force was only 0'05 inch of 

 mercury." 



Mr. Siemen's air-pump was exhibited by Knight and Sons. " It 

 consists of two cylinders of different diameters, the smaller one placed 



