HYDROMETER. 



alwaysbeen essleemed by philosophers, as 

 well as men of business. It consists of a 

 hollow ball, either of metal or glass, ca- 



Eable of floating 1 in any known liquid ; 

 om the one side of the ball proceeds a 

 stem, which terminates in a weight, and 

 from the side diametrically opposite pro- 

 ceeds another stem, most commonly of 

 an equal thickness throughout. The 

 weight is so proportioned, that the instru- 

 ment may float with the last mentioned 

 stem upright. In the less accurate hy- 

 drometer this stem is graduated, and 

 serves to show the density of the fluid, 

 by the depth to which it sinks, as the 

 heavier fluids will buoy up the instrument 

 more than such as are lighter. In this 

 way, however, it is clear, that the stem 

 must be comparatively thick, in order to 

 possess any extensive range ; for the 

 weight of vitriolic ether is not equal to 

 three-fourths of the same bulk of water, 

 and therefore such an hydrometer, in- 

 tended to exhibit the comparative densi- 

 ties of these fluids, must have its stem 

 equal in bulk to more than one -fourth of 

 the whole instrument. If this bulk be 

 given chiefly in thickness, the smaller dif- 

 ferences of density will not be percepti- 

 ble, and it cannot, with any convenience, 

 be given in length. 



To remedy this imperfection, various 

 contrivances have been proposed, for the 

 most part grounded on the consideration, 

 that a change in the ballast, or weight 

 employed to sink the ball, would so far 

 change the instrument, that the same 

 short range of gradations on a slender 

 stem, which were employed to exhibit 

 the densities of ardent spirits, might be 

 employed in experiments upon water. 

 Some have adjusted weights to be screw- 

 ed upon the lower stem, and others, with 

 more neatness and accuracy, have adjust- 

 ed them to be slipped upon the extremi- 

 ty of the upper stem. But the method of 

 Fahrenheit appears to be on all accounts 

 the simplest and most accurate. 



The hydrometer of Fahrenheit consists 

 of a hollow ball, with a counterpoise be- 

 low, and a very slender stem above, ter- 

 minating in a small dish. The middle, or 

 half length of the stem, is distinguished 

 by a fine line across. In this instrument 

 every division of the stem is rejected, and 

 it is immersed in all experiments to the 

 middle of the stem, by placing proper 

 weights in the little dish above. Then, 

 s the part immersed is constantly of the 

 same magnitude, and the whole weight of 



thehydrometeris known, this last weight, 

 added to the weights in the dish, will be 

 equal to the weight of fluid displaced by 

 the instrument, as all writers on hydro- 

 statics prove. And accordingly the spe- 

 cific gravities for the common form of the 

 tables will be had by the proportion. As 

 the whole weight of the hydrometer and 

 its load, when adjusted in distilled water, 

 is to the number 1,000, &c. so is the whole 

 weight, when adjusted in any other fluid, 

 to the number expressing its specific gra- 

 vity. 



In order to show the degree of accu- 

 racy an instrument of this kind is capable 

 of, it may in the first place be observed, 

 that the greatest impediment to its sensi- 

 bility arises from the attraction or repul- 

 sion between the surface of the fluid and 

 that of the stem. If the instrument be 

 carefully wiped with a clean soft linen 

 cloth, the metallic surface will be equally 

 disposed to attract or repel the fluid. So 

 that if it possess a tendency to descend, 

 there will be a cavity surrounding the 

 stem ; or if, on the contrary, its tendency 

 be to rise, the fluid will stand round the 

 stem in a small protuberance. The ope- 

 rator must assist this tendency by apply- 

 ing the pincers, with which he takes up 

 his weights to the rim of the dish. It is 

 very easy to know when the surface of the 

 fluid is truly flat, by observing the reflec- 

 ed image of the window, or any other fit 

 object seen near the stem in the fluid. In 

 this way the adjustment of the weights 

 in the dish may, without difficulty, be 

 brought to the fiftieth part of a grain. If, 

 therefore, the instrument displace one 

 thousand grains of water, the result will 

 be very true to four places ot figures, or 

 even to five. This will be as exact as 

 most scales are capable of affording 



Some writers have spoken of the ad- 

 justment of an hydrometer of this kind, 

 so that it shall at some certain tempera- 

 ture displace one thousand grains of 

 water, as if this were a great difficulty. It 

 is true, indeed, that the performance of a 

 piece of workmanship of this nature 

 would require both skill and judgment 

 on the part of the artist ; but it is by no 

 means necessary. 



Nothing more is required on the part 

 of the workman, than that the hydrome- 

 ter shall be light enough to float in ether, 

 and capable of sustaining at least one- 

 third of its own weight in the dish, with- 

 out oversetting in a denser fluid. This 

 last requisite is obtained by giving a due 



