290 



OF THE EQUILIBRIUM OF FLOATATION. 



Let AB be the beam of a balance very nicely equipoised upon its 

 centre of motion at c, and suspended 

 from the fixed object represented at F, 

 the centering being so delicately exe- 

 cuted, that the equilibrium of the instru- 

 ment is disturbed by the smallest por- 

 tion of a grain being added to or sub- 

 tracted from either arm of the beam. 



D and E are two scales, which, together 

 with their appendages are also balanced 

 with the greatest exactness ; one of them 

 as E having a hook in the middle of its 

 bottom surface, to which the weight w 

 is suspended by means of a horse hair, 



or any other flexible substance of such extreme levity, as to have no 

 sensible effect upon the equilibrium. 



p is an upright pillar placed directly under the centre of motion, 

 and carrying the circular arc mmm, which serves to prevent a too 

 great vibration on either side, and also, by means of the index i, which 

 is fixed on the beam immediately under the fulcrum, it indicates the 

 exact position of equilibrium ; for it is manifest, that when the beam 

 is horizontal, the pointer must be directly over the middle of the arc. 

 The pieces in the scale D, denote the weight of the body when 

 weighed in air ; but when the body is immersed in water, as repre- 

 sented by the figure abed, the scale D with its accompanying weights, 

 must evidently preponderate, and for the purpose of restoring the 

 equilibrium, small weights must be placed in the opposite scale at E ; 

 and since the weights thus added, indicate the weight of a quantity of 

 water of equal bulk with the immersed body, it follows, that the specific 

 gravity of the body can from thence be determined. 



The hydrostatical balance, like the hydrometer or aerometer pre- 

 viously explained, has undergone various alterations and improve- 

 ments, according to the ideas of the different individuals who have 

 had occasion to apply it in their inquiries respecting the specific 

 gravities of bodies ; but since the general principle is the same in all, 

 under whatever form the instrument may appear, it would lead to 

 nothing useful to enter into a detailed description of the various 

 improvements which it has received, and the numerous changes that 

 have been made upon it ; we shall therefore refrain from farther dis- 

 cussion on the nature of its construction, and proceed to exemplify the 

 manner in which it is applied to the determination of specific gravities. 



