[ 3i I 



not publifhed 'till upwards of thirty years after Don Ulloa's ex- 

 periments. The air then remaining in his barometer, which he 

 probably filled at Lima, in whofe territory Guancavelica lies, under 

 a preffu.re of twenty-eight or between twenty-feven and twenty- 

 eight (French) inches, muft have confiderably expanded and de- 

 preffed the column of mercury under it, when brought up a moun- 

 tain of perhaps 7000 or 8000 feet high; and hence this mercurial 

 column remained fo low at Guaricavelica as eighteen inches and 

 one line. 



That in thefe circumftances the air contained in the barometer 

 might caufe a difference of three inches or more between the 

 height at which mercury purged of air would ftand, appears by 

 the experiments of Caflini, Mem. Par. 1740, Sur la Merldienne 

 de Paris ^ p. clxxii. for he found that the mercury, freed from air by 

 ebullition in the tube, flood four or five lines higher than in baro- 

 meters filled without that precaution. Nay, Cardinal Luynes 

 found the difference betwixt fuch barometers to amount fometimes 

 to fourteen lines, Mem. Par, 1768, p. 490 in 8vo. How great 

 muft it therefore be in barometers tranfported to greater heights 

 than that at which they were filled ? 



Moreover, Don Ulloa exprefsly tells us that the mountain on 

 which thefe fhells were found was every where habitable, Mem. 

 Philofophiquesy p. 34 and 35, which it could not be, and would 

 be exprefsly contradided by Bouguer, if its height were 13,000 

 Englifh feet over the furface of the fea. From all which I con- 



VoL. VIII. E elude 



