[ 45 ] 



tancc. The marine barometer, if we may credit the moft ref- 

 pedable authorities, that of Dodor Halley *, Captain Middle- 

 ton f, Lord MulgraveJ, and many French navigators §, never 

 fails to indicate a ftorm feveral hours before-hand. But with 

 refpcd to lefs confiderable changes of weather, it muft be allowed 

 that its variations afford no indications abfolutely certain, though 

 with certain reftridions they afford forae ground for probable 

 conjedurc ; and the reafon of this difference is (as will more 

 plainly appear in the fequel) bccaufe the height of the mercury 

 in the barometer hath no immediate and neceffary connexion 

 either with rain or fair weather. That its variable height is the 

 immediate confequence of the variable preffure of the atmofphere 

 admits of no doubt, but the caufe or caufes of this variable pref- 

 fure have not yet been fully afcertained ; many indeed have been 

 fuggefted, but none that reached the full extent of the phasno- 

 mena. This acknowledged want of fuccefs of my predeceffors 

 in this enquiry, at the fame time that it entitles me to make 

 further refearches in this intricate fubjed, will, I hope, plead my 

 apology in cafe they prove equally unfuccefsful. In order to lay 

 the whole matter fully before the Academy, I fhall firft ftate the 

 principal obfcrvations made on the variations of this inftrument ; 

 fecondly, the principal caufes to which thefe variations have 

 hitherto been referred, with a few remarks to fhew the infuf- 



• rV. Phil. Tianf. Abr. Part II. p. 6. 



t VIII. Phil. Tranf. Abr. p. 468. 



% Phipps's Voyage, p. 74. 



J Encyclop. par Ordre des Matieres Navigation, torn. i. p. 112. 



ficiency 



