HYDRODYNAMICS. 



443 



fcmae- the surface of the mercury, when it stands between any 

 " two degrees of the thermometer, and the indication (or 

 ^ * numbers on the stem) by the nearest division belom the 

 surface of the sample, when its level cuts -the stem of 

 the hydrometer between one division and another, thus 

 giving the difference in favour of the trader in both 

 caves. The square weight or Cap shews the difference 

 between the weight of proof spirit and that of water, as 

 dtM^ihjil in the first clause of the hydrometer act, and 

 being one-twelfth part of the total weight of the hy- 

 drometer and weight 60. If this weight is placed up- 

 on the top of the stem at A, and if the hydrometer U 

 loaded with weight No. 6, it will sink in distilled wa- 

 ter at the temperature of 51 to the proof point P, at 

 that temperature, as marked on the narrow edge of the 



o. - 



I r T --..1 c 



PlATI 



etc xiv. 



|* *.!*. 



20. On the Hydrottalic Balance. 



Although the hydrostatic balance can scarcely be 

 called a hydrometer, yet as it is employed for measu- 

 ring specific gravities when great accuracy is required, 

 we shall give a description of it in this place. The 

 hydrostatic balance, of w hich we have given a front and 

 a side view in F.gs. 8. and 9, of Plate CCCXI V. is no- 

 thing more than a good balance, with some additional 

 apparatus for enabling it to measure specific gravities 

 with accuracy and expedition. The support of the ba- 

 lance is a pillar AB, fixed into a stand or base CD. By 

 aflken string P e. Fig. 9, attached to a screw S, 

 and passing over the pullies P. p, is saapanilfil the 

 balance by means of the horizontal arm EF, on which 

 the hook e is hung : a/. / t, are the arms of the ba- 

 lance, e/its tongue, c, d, the scales, and MM a slender 

 carved rest, which prevents any of the arms from de- 

 scending too far on either side. By means of the screw 

 S, a vertical ascent or descent of the balance can be ob- 

 tained ; but when it is necessary to raise it much high- 

 er or lower, the whole sliding-piece into which the 

 screw S is fixed, may be moved either forward or 

 backward in a groove, or upon a brass rod placed upon 

 the stand CO. A board GH is fixed upon the project- 

 ing arm L beneath the two scales, and being rooveable 

 up and down in a groove, it can be fixed in any posi- 

 tion by a screw, in the middle of the lover surface of 

 each tcale is fixed a hook, to which are suspended brass 

 wires ck, dg, which pass freely through two openings 

 in the boar : (ill To the wire c k is suspended a cy- 

 lindrical wire K k, about 5 inches long, perforated at 

 each end, and covered with paper, containing a scale of 

 equal parts. A brass tube MS is fixed on the corner 

 of the board GH, on which a wire MN is made to move. 

 On the lower part of the wire moves another tube W, 

 carrying an index NO. which can be moved either ho- 

 risontally by turning round the tube, or vertically by 

 it up or down ; so that the index MN can be 

 to point to any division on the scale K k. A 

 t k is suspended to the wire K k, to which is fixed 

 the wire*-/, (of such s site that one inch of it will 

 weigh about * grains,) with a small brass ball / about 

 I of an inch in diameter, 

 like manner saapei 



The length of these wins is amen, that the ball /, and 

 the babble A, hang about the middle of the cylindrical 

 glass vessels X, Y, in the ordinary position of the ba 

 lance. Since brass is nearly eight times heavier than 

 water, it is evident, that for every inch that the wire k I 



sinks in the water in the vessel X, it will become half a Hydromt- 

 grain lighter, and for every inch that it rises out of the .*"*_. 

 water it will become half a grain heavier : consequently v ""V" 

 if it sinks 2 inches below its middle point x, or rises two 

 inches above it, the wire will become one grain lighter or 

 heavier. Let the middle point x, tlierefore, be brought 

 to the surface of the water, and the index NO set to the 

 middle of the scale K k, and let the distances OK, Ok 

 be each divided into 10O parts, then if it is required to 

 weigh bodies to the accuracy of the 1 00th part of a 

 grain, it may be done in the following manner. Let 

 the body to be weighed be placed in the scale c, and 

 let its weight be between 52 and 53 grains as determi- 

 ned by the weights in the opposite scale, then if we 

 move the balance gently up and down by the screw S, 

 till die tongue of the balance ef indicates a perfect 

 equilibrium, the distance of the index NO from k, as 

 measured upon (he scale, will indicate the number of 

 hundredth parts of a grain which the real weight of 

 the body is above 52, or below 55, according as either 

 of these weights is placed in the scale d. If 52 be the 

 weight in the scale d, then since the weight of the body 

 in the scale c exceeds 52, the scale c will preponderate, 

 and the balance being let down until the equilibrium is 

 restored by the loss of weight sustained by the immer- 

 sed wire kl, the index NO will rise as it were from the 

 mid 'tie part of K * Hence if it points to 12 divisions 

 above the middle of K k, the weight of the body will be 

 52.12 grains Had the weight 53 been placed in the 

 scale, it would have been necessary to raise the balance, 

 so that the scale c might acquire an equilibrium with 

 53 grains, by an addition to the weight of the wire k I 

 in consequence of its ascent from the water. In this 

 case, the index NO would have pointed to division 8!} 

 below the middle point of the scale, and the weight of 

 the body would have been 53.00 0.8b = 5'Z1* as 

 before. 



The weight of the body in air being thus obtained 

 with the utmost accuracy, it is next to be suspended to 

 the hook g by means of the horse hair, and weighed 

 when immersed io the water in the jar Y. The differ- 

 ence between these two weights, when they have been 

 corrected by the methods formerly described, will lead 

 to an accurate determination of the specific gravity of 

 the body. The wire * / should always be oiled, and the 

 oil wiped off. so that a thin film may adhere to it, in or- 

 der to prevent the mfl**"^" of the water. 



A large glaw babble h, s n 

 by a horse hair to the wire dg. 



SECT. IL On I He Table of Specif c GravUiet. 



The determination of the specific gravities of bodies 

 is of the greatest ue, not only in many of the sciences, 

 but also in most of the practical arts of life. Hence it 

 has been the object of philosopher* to determine, with 

 all th accuracy in their power, the specific gravities of 

 the various solid and fluid substances which occur in 

 nature. The following Table, which we have collected 

 with great labour from the tables of Brisson and other 

 sources, contains the most important specific gravities 

 that have been determined. A ll the measures are related 

 to that of water, whose specific gravity U 1.000, exct-pt- 

 ting the gases or aeriform bodies, whose specific gravi- 

 ties are determined in relation to that of atmospheric 

 air. which U taken at 1.00. The bodies are arranged 

 in an alphabetical order for the sake of more eaty refe- 

 rence. 



BN Prep. XII. and XJIL p. 43*. 435. 



