of graduating Glass Hydrometers. 69 



of the fluid equal to itself in weight. Hence we derive a me- 

 thod 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 com- 

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

 another fluid of known specific gravity ; 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 di- 

 vided 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 

 taking the specific gravity of solids, and counterpoised by 

 weights in the opposite scale. If a vessel of water be placed 

 under the hydrometer, and weights taken from its counter- 

 poise, 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 

 hydrometer for acids and saline solutions, beginning with water, 

 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 the 

 instrument when finished ; then to find the place where any 



other specific gravity y should be marked, y : 1 :; x : — , then 



x being added to the counterpoise, the instrument will 



rise. The vessel should then be lowered a little to bring the 

 beam horizontal, and the mark cut by the water noted for 

 specific gravity y. 



In the same manner, by simply adding weights to the scale, 

 as many other specific gravities may be found as are thought 

 necessary ; when the scale is to be withdrawn and laid flat, and 

 the intervals measured with a pair of compasses, and trans- 

 ferred to a new scale, the true specific gravities written oppo- 

 site their proper marks, and the scale put in its place. The in- 

 strument may be loaded a few grains heavy, and nicely adjusted 

 in sealing. It will show true specific gravities without referring 

 to a table, temperature being attended to. 



If it is desired to adjust a hydrometer for spirits, or fluids 

 lighter than water, — then the scale will begin at the lower part 



of 



