BAUMÉ’S HYDROMETER.--Make a cylinder between two points, and solder it to the extremity of a tube with thin sides, and which must be very regular on the outside. Close the open part which is to form the stalk of the hydrometer with a little wax. See pl. 1, fig. 9 and 15. When the soldering, which must be well done, is complete, and the stalk well centered, choke the reservoir at a little distance from the base of the point, by drawing it out in such a manner as considerably to diminish the canal in this part. Remove then the ball of wax which closed the tube, draw off the point of the cylinder, and make the part which was pulled away from the cylinder by the choking, into a bulb, by blowing with precaution into the tube. If the reservoir is required to be spherical instead of cylindrical, it must be softened and expanded by blowing. When it is intended to ballast the instrument with mercury, the canal must be completely stopped at the point where it is choked. In this case, the part drawn away from the cylinder is expanded into a bulb by blowing through the extreme point, which is to be cut off after the instrument is completed. In the first case, you ballast the instrument with lead shot, which you fix in the lower bulb by means of a little wax, which closes the canal at the choked part. In the second case, after having proved the ballast by putting it first into the large reservoir, it is removed into the little bulb, and the latter is immediately sealed. One of the essential conditions of a good hydrometer is that the stalk should keep a perfectly vertical position when the instrument is plunged in water. If, therefore, on proving the ballast, you perceive the stalk to rest obliquely, you must take care, on retiring it from the water, to wipe it dry, and to present the choked part between the cylinder and the little bulb to the flame; when it is softened, it is easy, by giving it a slight bend in the direction where the stalk of the hydrometer passes from the vertical, to rectify the defect. Finally, when the instrument is ballasted, you must seal the stalk, after having fixed in its interior the strip of paper which bears the graduated division. This method of operation serves equally for all the areometers known under the names of _areometer of Baumé_, _pèse-sels_, _pèse-liqueurs_, _pèse-acides_, and _hydrometers_, which differ only in the scheme of their graduation. As to the _size_ and the _length_ of the stalks, they depend upon the _dimensions_ you desire to give to the degrees of the scale, and upon the _use_ to which the instruments are destined. For the areometer of Baumé, and for the _pèse-sels_, the stalks are generally thicker and shorter than for hydrometers. Pl. 4, fig. 19, 20, and 21, represent different hydrometers. * * * * *