Defcripiion of a ttev) Air-pump. 503 



■barrel, -within which cafe Aides the toothed bar, or rack, F, moved by the wheel, and this rack 

 ^(with its cafe) is fixed exaftly parallel to the axis of the cylinder, that it may draw out the 

 pifton rod precifcly in that direftion in which itfelf moves : the bar is -/g of an inch in 

 fhicknefs, but is an inch in breadth from the bottom of its teeth to the back of it. It is 

 ?nade thus ftrong, that the end of it may not be drained upward from the proper diredion, 

 when it is urged forward by the wheel, and yet checked by the pifton rod, after the pifton 

 is raifed to the top of the barrel. To confine the rack in the true line of its motion, it is 

 made perfedlly ftraight, and of the fame dimenfions in its whole length, and its cafe the 

 fame, fo as to fit each other moft exactly, that it may be kept in its due pofition, when the 

 greateft part of it is drawn out of the cafe ; for which purpofe alfo it is made (as likewifc 

 the cafe) fo much longer at either end than the part neceflary to be toothed, as to permit a 

 great part of it to remain in the cafe, when the pifton rod is drawn out to its utmoft extent : 

 accordingly in this pump it ads, in this refpeft, as well as could be wiflied*. A notch is 

 cut out of the cafe at I, to allow the teeth of the wheel ^o take into thofe of the rack ; and 

 to keep the cafe firmly in its place, little notches are cut in the upper edge of it, into which 

 the contiguous parts of the pillars are let, and it is fecured fo by wedges, 2 2, underneath. 

 It will be known that the cafe of the rack has its due pofition, when the arm G being taken 

 off, both the rack and the pifton rod pulled out to their limit, are found to be parallel. The 

 pillar^ are made as fliort as poflible j to favour which the frame M has a furrow cut in the . 

 middle of it, under the wheel I, to let the teeth of the wheel defcend almoft to the 

 table on which the machine refts \ and the pump barrel is placed as near as may be to the 

 rack. 



On the upper part of the box, containing the collar of leathers, is a projefting part of the 

 metal, in the fliape of a cube, forming a little pedeftal Qj on this is placed the valve y"; from 

 beneath which defcends a ftraight duft into the barrel, without penetrating the box : the 

 form of the parts of this valve (all made of brafs, and of the fize of the originals belonging 

 to the pump) is reprefented in fig. 4. in a vertical fedion. A A is a fquare plate (to be taken 

 off if it (hould require to be ground and poliflied anew In contadl with the valve), which is 

 fattened on the pedeftal with cement ; it has a hole in the middle, being the opening of the 

 du£l, in which is inferted the little elevated pipe c, to be occafionally taken out ; whofe ufe 

 is to prevent the oil applied to the valve from being blown down into the du£i: by the air ru(h- 

 ing into the barrel : the elevated part of the plate a, a, is circular, and has its upper furface 

 made plane and poliflied, on which refts the valve D j which is fo far hollowed within, that 

 only its lower edge, being about ^^'^th of an inch thick (which is alfo well poliflied), may be in 

 contad with the plate finder it, and alfo that its cavity may rife above the little pipe : the valve 



* By this contrivance of fixing the barrel of the pump horizontal, and its rack underneath the barrel, it is 

 made fo portable, that I have packed it (the gage-glafs and receiver being taken off) in a box two feet long, 

 eighteen inches wide, and fcven in depth ; and it Ihould be remembered that the moft operofe parts of it here 

 defcribed, are the frame and machinery neceffary to render a pump, with fo long a Cylinder, portable ; a great 

 p3rt of which machinery, if it were not portable, would not be fpared, but merely exchanged for the huge 

 fiame of tbofe fo conftrufted. — L. 



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