[The principal hydrometer now in use is that of Sykes, this is adopted in estimating excise duties on liquids. That of Baumé is principally employed abroad. Those of Beck or Cartier are but rarely used. These instruments differ merely in their graduation. Sykes’s plan of increasing the extent of the indications without enlarging the instrument is ingenious. It is effected by means of a number of weights which may be appended as collars to the stem of the instrument. A useful method of ascertaining specific gravities for commercial purposes, consists in using a series of glass beads, previously adjusted and numbered. When thrown into any liquid, the heavier ones sink and the lighter float on the surface; but the one which has the same density as the liquid will remain indifferent, or perhaps slightly below the surface. The specific gravity is then found by the number with which it is marked.] FOOTNOTES [413] A fuller account of Hypatia may be found in Menagii Histor. Mulier. Philosoph. Lugd. 1690; Bruckeri Hist. Crit. Philos. ii. p. 351; and Wolfii Fragmenta Mulierum Græc. Gott. 1739, 4to. [414] Varia Opera Mathematica D. Petri de Fermat, Tolosæ, 1679, folio. [415] Cogitata Physico-Mathem. Par., 1644, and in Phænomena Hydraulica. [416] On Georg. i. 109. These words are quoted by Emmenessius, the editor of the Variorum edition of Virgil, but in the edition of Servius, Venetiis, 1562, fol., they are not to be found. The Commentary of Servius may at present be no longer indispensable for explaining Virgil; but it deserves to be printed once more as completely and accurately as possible. It contains much useful information, as well as many fragments of works now lost; and on this account cannot well be entirely dispensed with. [417] Quæst. Nat. iii. 25, p. 726. [418] Hist. Nat. xxxi. 3, sect. 23, p. 552.--Athen. ii. p. 46.--Plutarchi Quæst. Nat. 7. [419] De Simplic. Med. Facultatibus, iv. 20. [420] De Natura eorum quæ effluunt ex Terra, lib. ii. p. 124. [421] Philosophia Experimentalis, sive Commentaria in Aristotelis Meteorolog. lib. ii. textus 26, quæst. 2, tom. ii. p. 158, b. [422] Mundus Subterraneus, vol. i. p. 254. [423] Cursus Mathemat. p. 455, icon. 20. [424] Collegii Experiment. pars ii. Norimb. 1715, 4to. [425] Nuovi Ritrovamenti. Roma, 1696, fol. [426] Philosoph. Transact. 1675: where an engraving is given of all the parts. [427] Journal des Observations Physiques et Math. Par. 1714, 4to. [428] Philosoph. Transact. No. 384, p. 140; and No. 413, p. 277. [429] Comment. Acad. Petrop. v. p. 274. [430] In his Versuchen. Halle, 1737, 8vo, i. p. 556. [431] Theatrum Hydrostaticum. LIGHTING OF STREETS.