Ni'W Injirtiment for Meafuring specific Gnavitiei. Ill 



wlilch might impede or favour t'liis motion. The fpeeific gravity of bodies muft ntceffarily 

 conftitute par: of thofe obfervatioiis : for it ferves not only to indicate the nature of bodies 

 under examination, but likewife affords information refpe£ling their purity, and the (late 

 of their aggregation, condenfation, or rarefaflion, which are alfo the immediate caafes of 

 divulfion or repofe. It is therefore of importance that the inftrumcnts for meafuring this 

 property of bodies fliould be brought to the utmoft perfeiSlion they are capable of receiving, 

 and that they fliould be rendered as convenient as poffible for habitual ufe. 



Of all the hydrometers hitherto known, that of Fahrenheit is acknowledged to be the mod 

 accurate. The well known principle of this intlrument is, that it afcertains the weight of a 

 conRant bulk of the feveral fluids under examination. Such hydrometers as are conltrutled 

 to meafure the denfity by the degree of immerfion", may ferve, in fome manufaSories, to 

 give an approximation fulliciently near for the purpofe required. But without confidering 

 the inequality of the ftem, the tedious work of graduating by obfervation,.and the unaer- 

 tainty of eftimate from one divifion to another, it may be remarked, that they are not capable 

 of correftion for the ditTerent temperatures. In a word, tliey are not ht for the hand cf the 

 philofopher. 



The form which Nicholfon gave fome years ago to the hydrometer of Fahrenheit *, ren- 

 dered it proper to meafure the denfity of folids. At prefent it is very much ufed. It gives, 

 with confiderable accuracy, the ratio of the fpeeific gravity to the fifth decimal, water being 

 taken as unity. It is fufceptible of correclion for the variations of temperature, and the 

 impurity of the water which it is fometimes more convenient to ufe, as may be feen in the 

 article Ai-eometer^ in the chemical part of the Encyclopedie Methodique. It does not ap- 

 pear that any better inftrument need be wilhed for in thi-; refpett. 



Hut this hydrometer has hitherto been con(lru£led in metal only ; fo that it could not be 

 applied either to falts or acids. It is known likewife that areometers conftrutted on t'le 

 principles of Fahrenheit, for fpirituous, faline, and acid liquors, require to be varied in 

 magnitude, form, and quantity of ballall. In the one kind, the lower weight muft be at a 

 great dillance from the buoyant part, to maintain the vertical fituation ; in the other, it is 

 brought nearer, to operate on fmaller raafles of the fluid.' Thofe inftruments which are 

 intended for alcohol muft be light, and thofe applied to determine the denfity of concen- 

 trated acids muft be heavy. The mafs and dimenfions of every inftrument of this nature 

 muft be adjufted in fuch a manner that the additional weight may operate not only as a 

 load, but as ballaft. It cannot be applied in the upper bafon without deranging the ver- 

 tical fituation. From all which circumftances it happens, that a coUeflion of thefe inftru- 

 ments would be required, to anfwcr every purpofe of experiment. As a remedy for part 

 of thcfe inconveniences, it was ingeniouflv propofed that the areometer fliould terminate be- 

 neath in a hook, to which were fufpended at pleafure certain glafs balls filled with mercury, 



'" From Lowthorpc's Abridgement of tlic Philofciiliical Tranfaflions, I. 604, or Poyle's Works, in cjuarto, 

 I.oiulon, 17721 IV. 104, it appears that ilic hydrometer was firft invented by Boyle, and defcribcd under tiic 

 name of a new Efiay Inftrument. It had a graduated ftem, and, by means of a ftirnip or clip underneath, it wa& 

 ;,pi<licd, »» perfcflly as a gradu.red inftrument could be, to afccrtain tlie fpeeific gravities of Ibhds as well as 

 fluids. Fahrenheit firft applied a di(h for weights at the top, to afccrtain the fpccinc gravities of fluids only ; as 

 mvj be fcen in Read and Gray's Abridgement of the Philofophical Tmnfaftions, Vol. VI. Part I. p. 194. My 

 inllrumcni, referred to by Citizen Guyion, is a combination of both. It is defcribcd in the id volume of rlis 

 M.iiii;lielUr Memoirs, and is rcprcfcmed in Plate \'l. Fi^. 1. N. 



and 



