2t'> 2\'to Admi.-.juycnunt nf the Vulnna (,/ Bi^dlis. 



as may be atlcil upon by the rtanrfarJ JiquiJ. The learned author of the memoir from 

 which the following extrncl is taken, hr.s propofed to remove this difficulty by plunging the 

 fubjc£lof experiments in common air or a gafeous fluid, by means of an inftruracnt h.: 

 cjUs a ftcreometer. 



If the capacity of a vefTcl, or, which is the fame thing, the volume of air contained in thr.t 

 veflel, be mcafurcd, when the vefiel contains air only, and alfo when the veflcl contains a body 

 whofc volume is required to be known, the volume of air afccrtained by the firft mcafure- 

 iiicnt, dedutling the volume afcerta-ned by the fecond, will be the volume of the body itfelf. 

 Again, if it be admitted as a law, that the volume of any mafs of air be invcrfely as the prcf- 

 futc to which it is fubjeded, the temperature being fuppofcd conftant, it will be eafy to 

 deduce, from the mathematical relations of quantity, the whole bulk, provided the diifcr- 

 encc between the two bulks under two known preflurcs be obtained by experiment. 



Let it be fuppofed, for example, that the fird preffure is double the fecond, or, which fol- 

 lows as a confequence, that the fecond volume of the air be double the firft, and that the 

 difference be fifty cubic inches, it is evident tliat the firfb volume of the air will likewife be 

 fifty cubic inches. The ftereonieter is intended to afccrtain this difference at two known 

 preflures. 



The inftrumcnt is a kind of funnel, AB,Fig. i, Plate XIV. compofcd of a capfule A in 

 which the body is placed, and a tube B as uniform in the bore as can be procured. The 

 upper edge of the capfule is ground with emery, in order that it may be hermetically 

 clofed with a ^lafs cover INI flightly greafed. A double fcale is parted on the tube, having 

 two fets of graduations ; one to indicate the length, and the other the capacities, as deter- 

 mined by expeiiment. 



When this inftrument is ufed,lt mud be plunged in a vcfTel of mercury with the tube 

 very upright, until the mercury rifes within and without to a point C of the fcale, fee Fig. 2. 



The capfule is then clofed with the cover, which being greafed will prevent all commu- 

 nication between the external air and that contained within the capfule and tube. 



In this fituation of the inftrument, in which the mercury ftands at the fame height within 

 and without the tube, the internal air is compreded by the weight of the atmofphere, which 

 is known and expreffcd by the length of the mercury in the tube of the common barometer. 



The inftrument is then to be elevated, taking care to keep the tube conftantly in the verti- 

 cal pofition. It is reprefefited in this fituation Tig. 2, fecond pofition. The mercury dcfcends 

 in the tube, but not to the level of the external furface, and a column DE of mercury re- 

 mains fufpcndcd in tl« tube, the height of which is known by the fcale. The interior air 

 is therefore lefs compreflcd than before, the increafe of its volume being equal to the whole 

 capacity of the tube from C to D, which is indicated by the fecond fcale. 



It is known therefore that the preffiircs are in proportion to the barometrical column 

 and to the fame column diminifhed by the fubtiaftion of DE. And the bulks of the air n\ 

 thtfe two ftates are inverfely in the fame proportion, and again the dilTerence between thefc 

 bulks is the abfolutc quantity left void in the tube by the fall of the mercury, from which 

 data by an eafy analytical procefs the following rule is deduced : Multiply the number which 

 exprefies the lefs prefTure by that which denotes the augmentation of capacity, and divide 

 the produ£l by the number which denotes the diiTerence of the prefTurcs. The quotient 

 will be the bulk of the air when fubjeft to the greater prcffure. 



' To 



