New hijlrument for Meafuring Specijic Gravities. 113 



It may be ufed, i. for folids. It is the hydrometer of Nicholfon, from which it differs 

 in no refpe£l. The only condition will be, as in his inftrument, that the abfolute weight 

 of the body to be examined fliall be rather lefs than the conflant additional weight, which in 

 this inftrument is five grammes (115 grains). 



2. For liquids of lefs fpecific gravity than water, the inftrument, without the additional 

 weight above, weighs about two decagrammes (459 grains) in the dimenfions before laid 

 down. It w'i^ould be cafy to limit its weight to the utmoft accuracy. We have therefore 

 the range of one-fifth of buoyancy, and confequently the means of afcertainlng all the in- 

 termediate denfities from water to the moft highly reftified fpirit of wine, which is known 

 to bear in this refpeiSt the ratio of eight to ten with regard to water. 



3. When liquids of greater fpecific gravity than water are to be tried, the conftant 

 weight being applied below, by means of the additional piece, which weighs about fix 

 grammes (138 grains), the inftruments can receive in the upper bafon more than four times 

 the ufual additional weight, without lofing the equilibrium of its vertical pofition. In this 

 flate it is capable of fliewing the fpecific gravity of the moft concentrated acids. 



4. It pofTefles another property common to the inftrument of Nicholfon, namely, that 

 it may be ufed as a balance to determine the abfolute weight of fuch bodies as do not 

 exceed its additional load. 



5. Laftly, the purity of the w^ater being known, it will indicate the degrees of rarefaftion 

 and condenfation in proportion to its own bulk. 



I have little to fay refpefling the conftrudlion of this inftrument. Every workman in 

 glafs who {hall once fee it, will be able to make it without difficulty. The additional piece 

 for the lower bafon will require fome attention to make it perfectly agree with the con- 

 ftant upper weight, as to the immerfion of the inftrument. But this objeft may, by careful 

 adjuftment, be afcertained with the utmoft certainty and accuracy. 



The bulb of glafs is for this purpofe drawn out to a fine point ; a fufficient quantity of 

 mercury is tlien introduced to fink it, and the aperture clofed with a raorfel of wax. The 

 bulb being then placed in the lower bafon of the inftrument, the upper bafon is to be loaded 

 until the mark on the ftem becomes accurately coincident with the furface of the water. 

 The fum of the weights added above is precifely equal to that of the quantity of mercury 

 iieceflary to be added to that in the glafs bulb ; which done, nothing more is neceflary 

 than to feal the point by fufion, taking care not to change its bulk. 



Though this inftrument is rather delicate in form, it has no other imperfe(fbion than 

 the natural brittlcncfs of the material, which muft neceflarily be ufed for experiments with 

 faline and acid liquors. For fix months paft I have made very frequent ufe of one of 

 thefe inftruments in the Polytechnic School, without any other inconvenience than that one 

 of the branches of the lower bafon was accidentally broken. 



Nothing more remains but to render it portable. I apprehend th.u this.objccl is fuf- 

 ficicntly fccurcd by means of a cafe in which all the delicate parts are fecured from 

 prclVurc, and the heavier parts fupported in fuch a manner as to refill the cxcefs of motion 

 they are capable of acquiring by virtue of their mafs. This laft circumftance is frequently 

 overlooked by fuch workmen as arc employed in the package of inftruments; whence it nc- 

 ccftarily follows, that fome ftrain or frafturc muft be produced when matters of very unequal 

 airnfiry arc cxpofcd to receive a common impulfc. 



V:l. I— June 1707. Q^ Xli'c 



