Desaguliers himself constructed a hydrometer of the ordinary type for comparing the specific gravities of different kinds of water (Desaguliers's _Experimental Philosophy_, ii. 234). In order to give great sensibility to the instrument, the large glass ball was made nearly 3 in. in diameter, while the stem consisted of a wire 10 in. in length and only (1/40)in. in diameter. The instrument weighed 4000 grains, and the addition of a grain caused it to sink through an inch. By altering the quantity of shot in the small balls the instrument could be adapted for liquids other than water. To an instrument constructed for the same purpose, but on a still larger scale than that of Desaguliers, A. Deparcieux added a small dish on the top of the stem for the reception of the weights necessary to sink the instrument to a convenient depth. The effect of weights placed in such a dish or pan is of course the same as if they were placed within the bulb of the instrument, since they do not alter the volume of that part which is immersed. [Illustration: FIG. 3.--Nicholson's Hydrometer.] The first important improvement in the hydrometer after its reinvention by Boyle was introduced by G. D. Fahrenheit, who adopted the second mode of construction above referred to, arranging his instrument so as always to displace the same volume of liquid, its weight being varied accordingly. Instead of a scale, only a single mark is placed upon the stem, which is very slender, and bears at the top a small scale pan into which weights are placed until the instrument sinks to the mark upon its stem. The volume of the displaced liquid being then always the same, its density will be proportional to the whole weight supported, that is, to the weight of the instrument together with the weights required to be placed in the scale pan. Nicholson's hydrometer (fig. 3) combines the characteristics of Fahrenheit's hydrometer and of Boyle's essay instrument.[2] The following is the description given of it by W. Nicholson in the _Manchester Memoirs_, ii. 374:-- "AA represents a small scale. It may be taken off at D. Diameter 1½ in., weight 44 grains. "B a stem of hardened steel wire. Diameter 1/100 in. "E a hollow copper globe. Diameter 2(8/10) in. Weight with stem 369 grains. "FF a stirrup of wire screwed to the globe at C. "G a small scale, serving likewise as a counterpoise. Diameter 1½ in. Weight with stirrup 1634 grains. "The other dimensions may be had from the drawing, which is one-sixth of the linear magnitude of the instrument itself.