GRAVITY. 



•ne, it is faid to be fptfifrnS^, er in fpecie, heavier : and 

 the wooden one fpec'if.c ally lighter. 



This kind of gravity fome call relati-ut ; in cppofition to 

 nhfolnte gravity, which increafcs in proportion to the quantity 

 or mals of the body. 



Gravity, laws rjf the fpccific, tind levity of bodies. — I. If 

 two bodies be equal in bulk, their fpecitic grarities are to 

 t-ach other as their abfolute gravities. Thus a body is faid 

 to be twice as heavy, fpecifitally, as another, if it have twice 

 its gravity under the fame bulk. 



Hence, the fpecific gravities of equal bodies are as their 

 denlities. 



II. If two bodies, compared togethr-r, be of the fame 

 fpecific gravity, their abfolute weights will be as their mag- 

 nitudes or bulks. 



III. The fpecific gravities of bodies of the farne weight 

 are in the reciprocal ratio of their bulks. Hence the maffes 

 ef two bodies of the fame weight are in a reciprocal ratio of 

 their bulks. 



IV. The fpecific gravities of two bodies are in a ratio 

 compounded of the direft ratio of the abfolute gravities, 

 and the reciprocal one of their bulks. Hence, again, the 

 fpecific gravities are as the denfities. 



V. The abfolute gravities of two bodies will be in a com- 

 pound ratio of their fpecific gravities and their bulks. 



VI. The bu ks of two bodies will be in a compound ratio 

 ef their abfolute gravities direttly, and their fpecific gra- 

 Tities inverfely, 



VII. A b.idy fpecifically heavier than a fluid, lofes fo 

 much of its weight therein as is equal to the weight of a 

 quantity of the fluid of the fame bulk 



For, fuppofe a cubic inch of lead immergcd in water; a 

 eubic inch of v,ater will, thereby, be expelled from its place : 

 but the -weight of this water was fuft;ained by the rellllance 

 ef the ambient water. Tiierefore, fuch a part of the weight 

 of the leaden cube mull be fuftained by the refiftance of the 

 ambient water, as is equal to the weight of the water ex- 

 pelled. The gravity of the body immerged, therefore, mull 

 be diminiflied by fo much. 



Hence, i. Since a fluid, fpecifically heavier, has a greater 

 weight, in the fame bulk, than a lighter ; the fame body 

 ■will lofe a greater part of its weight in a fluid, fpecifically 

 heavier, than in a lighter : and therefore it weighs more in 

 A lighter than a heavier. 



2. Equal homogeneous bodies weighing equally in air, 

 lofe their equilibrium if one of them be immerged in a heavier 

 fluid ; the other in a lighter. 



3. Since the foecific gravities arc as the abfolute gravities 

 under the fame bulk ; the fpecific gravity of the fluid will 

 be to the gravity of the body immerged, as part of tlie 

 weight loft by the folid, to the whole weight. 



4. Two folids, equal hi bulk, lofe the fame weight in the 

 fame fluid : but the weight of the fpecifically heavier body 

 is greater than that of the fpecifically lighter; therefore, tiie 

 fpecifically lighter lofes a greater part of its weight than the 

 fpecifically heavier. 



5. Since the bulks of bodies, equal in weight, areiecipro- 

 caUy as the fpecific gravities ; the fpecifically lighter lofes 

 more weight in the fame fluid than the heavier ; wherefore, 

 if they be in cquilibrio in one fluid, they will not be fo in 

 another ; but the fpecifically heavier will preponderate, and 

 that the more, as the fluid is denfer. 



6. The fpecific gravities of fluids are as the weights lofl; 

 bv the fame folid immerged in the fame. 



VIII. To Jind the fpecific gravity of a jliiid. — On one arm 

 of a balance lufpend a leaden globe ; and to the other, faften 

 a weight, which i$ in equilibrium therewith in the air. Im- 



merge tlie glote fuece-ffivcly m the fcveral fluids, whofe fpe- 

 cific gravities are to be determined, and obferve the weight 

 which balances it in each. Thefe feveral weights, I'ubtra ;te<i, 

 fev^rally, from the firil weight, the remainders are the parts 

 of the weight loit in each fluid. Whence the ratio of the 

 fpecific gravity of the fluids is feen. 



Hence, as the denfities are as the fpecific gravities, we 

 find the ratio of the denfities of the fluids at the fame time. 



This problem is of the utmoil ufe : as by it the degree of 

 purity or goodnefs of fluids is eafily found ; a thing not 

 only of fervice in natural philofophy, but alfo in common 

 hfe, and in the pradtice of phyfic. See Hydrometer. 



That the fpecific gravity may be found the more accu- 

 rately, the weight of the thread not immerged in the fluidf 

 is to be fubtradled from the weight of the folid in air ; and 

 the force neceflary to make the thread fubfide (if it be fpe- 

 cifically hghter) is to be added to tlie weight loft. But if 

 the thread that fuftains the folid be heavier than the fluid, 

 the weight of the thread in the air is to be fubtrafted from 

 the weight of the folid in air ; and the weight the thread 

 lofes from the-weight loft in the fluid. Indeed, this pre- 

 caution may be fpared, if, in examining the fpecific gravity 

 of feveral fluids, care be taken that the fame thread be im- 

 merged to the fame depth in each. 



IX. To determine the ratio which the fpecific gravity of 

 a fluid has to the fpecific gravity of a folid that is fpecifically 

 heavier than the fluid, 



Weigh any mafs of the folid in a fluid, and note the juft 

 weight therein : the fpecific gravity of the fluid will be to 

 that of the foHd, as the part of the weight loll by the folid' 

 to its whole weight. 



X. The fpecific gravities of equally heavy bodies are 

 reciprocally as the quantities of weight loft in tlie fame fluid. 

 Hence we find the ratio of the fpeciilc gravities of folidi;, bv 

 weighing mafles thereof, that are equal in air, in the fame 

 fluid ; and noting the weights loft by each. 



XI. A body, fpecifically heavier, defcends in a fluid 

 fpecifically lighter, with a force equal to the excefs of its 

 weight, above that of an equal quantity of the fluid. 



Hence, I. The force which fuftains a fpecifically heavier 

 body in a fluid, is equal to the excefs of the abfolute gra- 

 vity of the body, above that of the fluid, under the fame 

 bulk : e. gr. 47J pound of copper loies jf- pounds of its 

 weight in v>'ater ; therefore a pov.'er of 42 pounds is able to 

 fuftain it. 



2. Since the excefs of the weight of a folid over the 

 weight of a fluid fpecifically heavier, is lefs than that 

 over the weight of a fpecifically lighter fluid under the 

 fame bulk ; it will defcend with Itfs force in a fpecifi- 

 cally heavier fluid than in a lighter ; and, confequently, 

 it v.ill delccnd more flowly in the former than in tlie 

 latter. 



XII. A fpecifically lighter body finks in a heavier fluid, 

 till the weight of a quantity of the fluid, equal in bulk to 

 the part immerfed, be equal to the weight of the whole 

 body. 



hence, i. Since the fpecific gravities of bodies of tlie 

 fame weight are reciprocally as their bulks ; and the 

 bulks of fluids equal in weight, are as the parts of th« 

 fame folid immerged therein ; the fpecific gravities of fluid* 

 are reciprocally as the parts of the fame body immerged 

 therein. 



2. A folid, therefore, immerges deeper in a lighter fluid 

 than in a heavier, and deeper, as the proportion of the fpe- 

 cific gravity of the folid to that of the fluid is greater. 



3. If a body be of the fame fpecific gravity with a fluid, 



the 



