246 Description of a Self-registering Hydrometer. 



the modification of it which I am now to propose, the principle 

 is the same, though the form is somewhat different. 



CD (fig. I.) is a tube, such as is commonly used for con- 

 structing a self-registering thermometer, bent upwards at C and 

 D, and t'^rminating in a bulb A. Into this bulb is introduced 

 a portion of sulphuric acid, sufficient to fill the tui)e and a small 

 part of the bulb ; and along with the acid a small bit of glass a, 

 of such a diameter as to move easily in the tube when the instru- 

 ment is inverted. To the extremity, D, another bulb, B, is at- 

 tached ; and the air contained in both bulbs is so adjusted, that 

 when they are at the same temperature, the liquid stands at a 

 point near the extremity D, and which is marked on the at- 

 tached scale E F. If the temperature of the bulb B be now in- 

 creased, or, which is the same thing, if that of A be diminished, 

 the portion of air in the upper part of the bulb will contract, 

 while that contained in B will expand in the same proportion, 

 and the liquid will of course be forced from D towards C. In 

 the scale adopted by Professor Leslie, the distance between the 

 freezing and boiling points is divided into a thousand equal parts, 

 and is hence denominated the miUesimal. In this climate, how- 

 ever, a tenth part of that scale, or one hundred degrees, will em- 

 brace the greatest range of the instrument; and that point may 

 be thus obtained: — Let the bulb A be surrounded with melting 

 snow, while the instrument is placed in an atmosphere of the 

 temperature 50, and let the point be marked at which the liquid 

 becomes stationary. The distance between zero and this point 

 will then be IS degrees of Fahr. or 100 of the millesimal scale; 

 and that distance being divided into an hundred equal parts, will 

 give the graduation required. To prepare the instrument for 

 observation, it only remains to cover the bulb A with silk, and 

 moisten it, taking care that the two bulbs be as nearly as possi- 

 ble of the sa'ue colour. The index, or small bit of glass, a, is 

 then to be brought to the extremity of the liquid, by depressing 

 the extremity D, and the instrument to be exposed in a hori- 

 zontal position. As the evaporation from the surface of the 

 bulb A goes on, the air within contracts, from the depression of 

 temperature produced by the evaporation ; and the liquid is 

 forced from D towards C by the elasticity of the air in B, car- 

 rying with it the index a. When the evaporation has reached 

 its maximum, the liquid, as well as the index^ becomes stationary; 



but 



I 



