ProjeBforanAirPiimp.—UfefulNotu-is. 13 r 



The moft perfe£t fcheme for an air pump, taking advantage of the labours of thefe ju- 

 dicious operators, feems to be that in which two pirtonsof the conftru£lion of Prince fhould 

 work in one barrel ; one pifton being fixed at the lower end of the rod, and the other at 

 the middle. The lower pifton muft come clear out of the barrel when down, and work 

 air-tight through a diaphragm at an equal diftance from the effeOive ends of the barrel. 

 In the diaphragm muft be a metallic valve of the form of Cuthbertfon's lower valve, but 

 with a fliort tail beneath, that it may be mechanically opened when the pifton comes up. 

 Above the diaphragm, mull work the other pifton fimilar to the firft ; but as it cannot quit 

 the barrel when down, a fmall. portion of the barrel muft be enlarged juft above the 

 diaphragm, fo that the leathers may be clear in that pofition. Laftly, the top of the barrel 

 muft be clofed and fitted with a valve and oil veflTel, according to the excellent contrivance of 

 Cuthbertfon. 



If we_fuppofe the workmanfliip of fuch a pump to leave the fpace between the diaphragm 

 and lower pifton, when up, equal to one-thoufandth part of the fpace paffed through by the 

 ftrokc of that pifton, the rarefadlion produced by this part of the engine will in theory bear 

 the fame proportion to that of the external air. And the fame fuppofition applied to the 

 upper pifton would increafe the efte£l one thoufand times more. Whence the rarefadlion 

 would be one million times. How far the pra£lical efFeft might fall fliort of this from, the 

 imperfeflions of workmanfliip, or the nature of the air, which in high rarefaftions, may not 

 difFufe itfelf equally through the containing fpaces, or from other yet unobferved circum- 

 ftances, cannot be deduced from mere reafoning without experiment. 



VI. 



Ufifiil Notices refpeBing various OhjeBs. — A Method of preventing Heat in Grinding — Con- 

 cerning Goldt Silvery and other Aietals reduced into very thin Leaves by the Hammer — 

 Globules for Microfcopes — On the Plumb-Line, and Spirit-Level. 



A 



I . On the Art of Grinding. 



CURIOUS fa£l was mentioned to me, fome years ago, refpefting grinding, which 

 promifes to be of fome ufe in the arts. Daily experience, as well as philofophical ex- 

 periment, fliews us that heat is produced or developed by fridion. The fa£l of fparks 

 flying from a dry grindftone when a piece of iron or fteel is applied to its furface during 

 the rotation, has been feen by every one. The heat produced during this procefs is fuch 

 that the fteel very foon becomes ignited, and hard tools are very frequently foftened and 

 fpoiled, for want of care during the grinding. When a cylindrical ftone is partly immerfed 

 in a trough of water, the rotation muft be moderate and the work (low, otherwife the 

 water would foon be thrown oft' by the centrifugal force ; and when this fluid is applied by 

 a cock from above, the quantity is too fmall to preferve the requifite low temperature. It is 

 even found, that the point of a hard tool, ground under a confiderable mafs of water, will 

 be foftened if it be not held fo as to meet the ftrcam ; fparks being frequently afforded 

 even under the water. My informant afturcd me, that fine cutlery is ground in Germany 

 on a cylinder of a peculiar kind of pottery inftead of ftonc, upon the face of which puU 



S 2 vcrifcd 



