Besides the above, many hydrometers have been employed for special purposes.
The marine hydrometers, as supplied by the British government to the royal navy and the merchant marine, are glass instruments with slender stems, and generally serve to indicate specific gravities from 1.000 to 1.040.
The various devices which have been adopted to overcome this difficulty will be described in the account given of the several hydrometers which have been hitherto generally employed.
The plan commonly adopted to obviate the necessity of inconveniently long stems is to construct a number of hydrometers as nearly alike as may be, but to load them differently, so that the scaledivisions at the bottom of the stem of one hydrometer just overlap those at the top of the stem of the preceding.
By this means a set of six hydrometers, each having a stem rather more than 5 in.
Perhaps the main object for which hydrometers have been constructed is the determination of the value of spirituous liquors, chiefly for revenue purposes.
To this end an immense variety of hydrometers have been devised, differing mainly in the character of their scales.
C. 140, by which it was enacted that "` all spirits shall be deemed and taken to be of the degree of strength which the said hydrometers called Sikes's hydrometers shall, upon trial by any officer or officers of the customs or excise, denote such spirits to be."