HYDROSTATICS. 



141 



What is a Hy- 

 drometer ? 



How may the 



specific grav- 

 ity of a liquid 

 be determined 

 by the Hy- 

 drometer 7 



319. The Hydrometer con- Fig. 127. 

 sists of a hollow glass tube, 

 oa the lower part of which a spherical 

 bulb is blown, the latter being filled with 

 a suitable quantity of small shot, or 

 quicksilver, in order to cause it to float, 

 in a vertical position. The upper part 

 of the tube contains a scale graduated 

 into suitable divisions. (See Fig. 127.) 



It 13 obvious that the hydrometer 



will sink to a greater or less depth in 



difierent liquids ; deeper in the ligliter 



ones, or those of small specific gravity, 



and not so deep in those which are 

 denser, or which have great specific gravity. The 

 specific gravity of a Uquid may, therefore, be estimated by the number of di- 

 visions on the scale which remain above the surface of the hquid. Tables 

 are constructed, so that, by their aid, when the number on the scale at which 

 the hydrometer floats in a given liquid is determined by experiment, the spe- 

 cific gravity ia expressed by figures in a column directly opposite that number 

 in the table. 



There are various forms of the hydrometer especially adapted for determin- 

 ing the density, or specific gravity, of spirits, oils, syrups, lye, etc. It afibrds 

 a ready method of determining the purity of a liquid, as, for instance, alco- 

 hol. The addition of water to alcohol adds to its density, and therefore in- 

 creases its buoyancy. The addition of water, therefore, will at once be shown 

 by the less depth to which the hydrometer will sink in the liquid. The 

 adulteration of sperm oil with whale, or other cheaper oils, may be shown in 

 the same manner. 



320. For the reason that the buoyancy of a liquid is proportioned to its 

 density, a ship will draw less water, or sail lighter by one thirty-fifth in the 

 heavy salt water of the ocean, than in the fi-esh water of a river ; for the 

 same reason it is easier to swim in salt than in fresh water.* 



• " A floating bodr sirks to the same depth whether the mass of liquid supporting it 

 be great or small, as is seen when an earthen cup is placed first in a pond, and then in a 

 second eup only so much larger than itself, that a very small quantity of water will suffice 

 to fill up the interval between them. An ounce of water in this way may be made to float 

 substances of much greater weight. And if a large ship were received into a dock, or 

 case, so exactly filling it that there were only half an inch of interval between it and the 

 wall, or side of the containing space, it would float as completely when the few hogsheads 

 of water required to fill this little interval up to its usual water-mark were poured in, as 

 if it were on the high seas. In some canal locks, the boats just fit the place in which they 

 have to rise and fall, ahd thus diminish the quantity of water necessary to Eupply th« 

 loci.."— A niott. 



