GRA 



GRA 



drometer, because these latter terms are 

 grounded upon the supposition that a fluid 

 is always the thing weighed; whereas with 

 regard to solids, the liquid is the known 

 term of comparison to which the un- 

 known weight is referred. Guyton's 

 gravimeter is executed in glass, and is 

 of a cylindric form, being that which re- 

 quires the smallest quantity of fluid, and 

 is on that account preferable, except so 

 far as it is necessary to deviate for the se- 

 curity of a vertical position. It carries 

 two basins, one of them superior, at the 

 extremity of a thin stem, towards the 

 middle of which the fixed point of immer- 

 sion is marked. The other or lower 

 basin terminates in a point ; it contains 

 the balls, and is attached to the cylinder 

 by two branches. The moveable sus- 

 pension by means of a hook has the in- 

 convenience of shortening the lever 

 which is to secure the vertical position. 

 The cylinder is three fourths of an inch 

 in diameter, and 6.85 inchesin length. It 

 carries in the upper basin an additional 

 constant weight of five grammes, or one 

 hundred and fifteen grains. These di- 

 mensions might be increased so as to ren- 

 der it capable of receiving a much more 

 considerable weight; but this is unne- 

 cessary. M. Guyton has added a piece 

 which he calls the plonguer, because, in 

 fact, it is placed in the lower basin when 

 used, and is consequently entirely im- 

 mersed in the fluid. It is a bulb of glass 

 loaded with a sufficient quantity of mer- 

 cury, in order that its total weight may 

 be equal to the constant additional weight 

 added to the weight of the volume of 

 water displaced by this piece. It will be 

 readily understood,that,the weight being 

 determined at the same temperature at 

 which the instrument was originally ad- 

 justed, it will sink to the same mark on 

 the stem, whether it is loaded with a con- 

 stant additional weight in the upper basin, 

 or whether the effect of this weight be 

 produced by the additional piece in the 

 lower dish. From this explanation there 

 will be no difficulty in seeing how this 

 instrument may be adapted to every case 

 in practice. It may be used, 1. For solids. 

 The only condition will be, that the abso- 

 lute weight of the body to be examined 

 shall be rather less that the constant ad- 

 ditional weight, which in this instrument 

 is about 115 grains. 2. For liquids of less 

 specific gravity than water, the instru- 

 ment, without the additional weight a- 

 bove mentioned, weighs about four hun- 

 dred and fiffty -nine-grains, in the dimen- 

 sions before laid down. It would be easy 



to limit its weight to the utmost accuracy. 

 We have therefore the range of one- 

 fifth of buoyancy, and consequently the 

 means of ascertaining all the intermediate 

 densities from water to the most highly 

 rectified spirit of wine, which is known 

 to bear in this respect the ratio of eight 

 to ten with regard to water. 3. When 

 liquids of greater specific gravity than 

 water are to be tried, the constant weight 

 being applied below by means of the ad- 

 ditional piece, which weighs about one 

 hundred and thirty -eight grains, the in- 

 strument can receive in the upper basin 

 more than four times the usual additional 

 weight, without losing the eqailibrium of 

 its vertical position. In this state it is 

 capable of shewing the specific gravity of 

 the most concentrated acids. 4. It pos- 

 sesses another property, namely, that it 

 may be used as a balance to determine 

 the absolute weight of such bodies as do 

 not exceed its additional load. 5. Lastly, 

 the purity of the water being known, it 

 will indicate the degrees of rarefaction 

 and condensation in proportion to its own 

 bulk. To find the specific gravity of any 

 solid by the gravimeter, observe this rule: 

 " From the weight in the upper dish, 

 when the instrument is properly immers- 

 ed in the unknown fluid, take the weight 

 which is placed with the body in the same 

 scale at the like adjustment. The re- 

 mainder is the absolute weight of the 

 solid. Multiply this by the specific gra- 

 vity of the fluid/and reserve the product. 

 From the additional weight, when the 

 body is placed in the lower basin, take 

 the weight when it was placed in the up- 

 per. The remainder will be the loss of 

 weight by immersion. Divide the reserv- 

 ed product by the loss by immersion, and 

 the quotient will be the specific gravity of 

 the solid with regard to distilled water at 

 the standard temperature and pressure." 

 To find the specific gravity of a fluid, 

 proceed thus: "To the weight of the 

 gravimeter add the weight required in 

 the upper basin to sink it in the ; unknown 

 fluid." Again, " To the weight of the 

 gravimeter add the weight required in 

 the same manner to sink it in distilled 

 water. Divide the first sum by the latter, 

 and the quotient will be the specific gra- 

 vity of the fluid in question." See SPE- 

 CIFIC GRAVITY, HYDROSTATICS, and HT- 



DROMETKR. 



GRAVING. See ENGRAVING. In sea 

 affairs the word graving is used for the act 

 of cleaning a ship's bottom, when she is 

 laid aground during the recess of the tide. 

 See BREAMING and CAREENING, 



