1826.] Mr. Moore on graduating glass Hydrometers. 261 



Article VI. 



A simple Method of graduating glass Hydrometers. 

 By Charles Moore,"Esq.* 



As hydrometers of glass are irregular in shape, they are 

 usually graduated by immersing them in fluids of different 

 specific gravities. But as a considerable number of fluids are 

 required, and as they are liable to change by evaporation a 

 different method may be found useful. ' 



In trying the specific gravities of fluids by a bottle of known 

 capacity, we compare together the weights of equal volumes ; 

 but in using a hydrometer we compare the volumes of equal 

 weights; as the instrument sinks until it displaces a volume of 

 the fluid equal to itself in weight. Hence we derive a method 

 of graduating a hydrometer by the help of one fluid only. 



Water, being the standard, "is the most convenient, and as its 

 specific gravity is supposed to be unity, we can easily compute 

 how much water is equal in volume to a given weight of another 

 fluid of known specific g'-avity ; or in other words, with what 

 weight a hydrometer should be loaded, in order to make it sink 

 in water at 60°, to the point where that specific gravity should 

 be marked : the weight of such hydrometer when finished being 

 determined. ° 



Let the hydrometer be loaded until it would, if permitted sink 

 entirely in water, and place in the stem a paper scale divided 

 into small equal parts^ taking care that some one known mark 

 corresponds with some remarkable part of the stem. Let it 

 then be suspended from a good small balance, as in takino- the 

 specific gravity of solids, and counterpoised by weights in the 

 opposite scale. If a vessel of water be placed undeAhe hydro- 

 meter, and weights taken from its counterpoise, it will of course 

 sink and displace an equal weight of water; and in this simple 

 and easy manner we can find the proper places for any required 

 specific gravities ; which may be written on a new scale and 

 put into a similar position. ' 



For example, suppose it were desirable to make a glass hydro- 

 meter for acids and saline solutions, beginning witlfwater and 

 running upwards as high as the length of the stem would allow • 

 suppose also, that the hydrometer when immersed to the upper 

 end of the stem was found to displace x grains of water- it is 

 plain that X grains should be the weight of llie instrument when 

 finished : then to find the place where any other specific gravity 

 3/ should be marked, y : 1 :: r : -, then x - - being added 

 to the counterpoise, the instrument will rise. Tlie vessel should 



• From the Dublin Philosophical Journal. 



