GRAVITY. 



the ■«-holc body will be immerged ; and it will remain in any 

 given place of the fluid. 



4. If a fpecificaily liglitcr body be wholly immerged in a 

 fluid, it will be urged, by the collateral columns of the 

 fluid, to afcr^^d vn-h a force equal to the excefs of the 

 weight cf the iUiid, bulk for bidk, over the weight of the 

 folid. 



cubic foot or cubic inch. As it would be rery difficuUi 

 and in many cafes impolTiblc, to meafurc the exact volume re- 

 quired to be weighed, a method of determining the fpecific 

 gravity of fohds has been dcvifed, founded on a principle of 

 Hydrojlatict. It appears to have been known to Arciiimedcs, 

 that when a folid body, heavier tlian water, is plunged into 

 that fluid, it lofcs as much of its weight as is equal to tlie 



5. A body, therefore, fpecifically lighter, lying on the weight of the fluid it has difplactd. 

 bottom of a Tcfiel, will not be raifed up unlefs the heavier By the application of this principle we are enabled, in > 

 .fluid nfe above fuch a part as ,s equal in bulk to a quantity very eafy manner, to determine the fpecif.c gravity of a 

 of the fluid of the fame weight with the wnole folid. body by f.rft weighing it in air and afterwards in water. 



XIII. The fpecific gravity of a folid is to the fpccifie Then if the weight in air be divided by the weight loft, 

 gravity of a' lighter fluid, wherein it is immerged, as the or the difference of the weights in water and air, the quo- 

 bulk of the part iir.merged is to the whole bulk. tient will exprcfs the fpeciiic gravity of that body, or the 

 . XIV. The fpecific gravities of equal folids are as their ''clative wciglit of an equal volume of that fubilancc, and of 

 parts immerged iii the fame fluid. ^'"^ water in which the experiment was made. 



XV. The weight and bulk of a fpecifically lighter body, '^''^ fpecific gravity of two fluids may be determined by 

 and the weight of the fpecifically heavier fluid, being given, ''"^' ^^^^ principle ; for if vve weigh a folid of any magni- 

 to find the force required, to keep the folid wholly im- *""?'^' ^^ ^ ^'■^ '^{ glafs, firfl in water and then in any other 

 merged under the fluid. S^md, the quantities of weight loll in each experimtut will 



As this force is equal to the excefs of the weight of the ^- .'" '"^ '^"""^ proportion as the fpecific gravity of the tv.-6 

 fluid bevond that of an equal bulk of the folid; from the ^^^' 



given bulk of the folid, and the weight of a cubic foot of J^^^ fpecific gravities of any fubftances, and in particular 

 water, find, by the rule of three, the weight of a bulk of . ^^^^ ^^ ^'■': '"'ghter than water, may alfo be very conve- 

 \»ater equal to that of the body. From this fubtrad the ""^"'b' determined by means of a common balance, employ- 

 weight of the foLd; the remainder is the force required. '='g a pJ"al «itl» a conical ground ftopple filling it firll with 

 E.gr. Suppofe the force neceffary to detain a folid eight 

 feet in biilk, and ico pounds in weight, under water, re 

 quired: fince a cubic foot of, water is found to weigh 70 

 pound, the weight of water under the bulk of eiglit feet, 

 is 560; whence, ico pounds the weight of the folid, being 

 fubtrafted; the remainder 460 pound is the force neceffary 

 to detain the folid under water. 



Hence, fince a fpecifically lighter body afcends in a 

 heavier fluid, with the fame force that would prevent its 

 afcent: by the prefent problem, we can likewiie find the 

 force wherewith a fpecifically lighter body afcends in a 

 leavier. 



XVI, The weight of a vefill, to be made of a fpecificrlly 



water and then either with a given fluid, or with a portion 

 of tlie folid of which the weight has been afccrtained, to- 

 getlier w ith as much water as is fufficient to exclude the air. 



It is neceflary to attend to a great number of minute cir- 

 cumfl;ances when we wifh to determine tlie fpecific gravities 

 of fubftances to a great degree of accuracy : thefe will be 

 particularly noticed when dcfcribing the particular inftru- 

 ments that have been invented for the purpofe. See Hv- 

 DRO.ST.\Tic Balance, Hvdro.mkter, &c. 



One mode of afcertaining the fpecific gravities of fluids, 

 differing but little from each other in denfity, is to have a 

 feries of globules of glafs fo loaded, as to correfpond to 

 the fpecific gravities indicated by as many numbers, wliich 

 heavier matter ; and that of a fpecifically lighter fluid, being at"^ marked on them, and throwing feveral of them toge- 

 given : to determine the cavity the vefiel muft have to fwim l''cr into the fluid, and to obferve which of tiiem remains 

 on the fluid. nearly llationary, v. ithout either rifing to the furface or 



The weight of a cubic foot of the fluid being given, the fii^king. This method, though not expedition?, appears to 

 bulk of the fluid equal to the weight of the veflel is found be very fecure from error, and is well adapted to determine 

 by the rule of three. If, then, the cavity be made a httle tj'e ftrength of fpirituous hquors. But in all thefe expe- 

 bigger than this, the veflel will have lefs weight under the riments it is neceflary to be aware that a confiderable change 

 fame bulk than the fluid, and will therefore be fpecifi- of ''le joint bulk of two fubftances is often prt)duced by 



cally lighter than the fame, and confequently it will 

 fwim. 



XVII. The force employed to retain a fpecifically hghter 

 folid, under a heavier fluid ; and the weight loft by a heavier 



their mixture, and that in general their dimcnfions arc con- 

 Cderably contracted. Thus, 18 gallons of water and 18 

 gallons of alcohol, iaftead of 36 gallons, make only jr, 

 confequently the fpecific graiity of the compound is one 



folid in a lighter fluid, are each added to' the weight of the 35th greater thaii the mean of the fpecific gravities of the 

 -•■ . ° . . . ... •= ingredients. And in lome cales the w hole dimenfion of a 



fluid, and weigh together with it 



The fevcml theorems here delivered, arc not only all de- 

 monftrable from the principles of mechanics, but are conform- 

 able to experiment. In effetl, experience is here found to 

 anfwer exaftly to calculation, as is abundantly evident from 

 the courfes of philofophical experiments, now frequently ex- 

 hibited ; where the laws of fpecific gravitation are well illuf- 

 trated. See Hydrost.\tical balance. 



fingle fubftance may even be contracted by the addition of 

 another fubftance ; thus iron, by the addition of one-eighth 

 of its bulk of platina, becomes contracted one-fortieth of 

 that bulk. 



For mcafuring the fpecific gravity of gafTis, Mr. Leilie 

 has devifed a new method, which eonfifts in obferving the 

 time employed in emptying a veffli through a fmall orifice, 



The fpecific gravity of any fubftance is the relative by means of the prelTure of an equal column of water, 



weight of a given volume of it, compared with the fame 

 Tolume of fome other fubftance taken as a ftnndard, and 

 which is ufually diftilled water at fome given tempera- 

 ture. The abfolute gravity of a body is its entire weight, 

 its fpecific gravity is the weight of a given portion, as a 



4 



A Table of Specific Gravil'ui. 



Principally from Davics and Lavoifier. Davles's table it 

 compiled with great diligence from many diifercnt authors ; 

 Lavoilitr's is chiefly extraded from BriiTen ; it is carried 



