2^S Mt-moir on i!?c Quantity of 



taken while the mercury of the barometer was ver)- low, and 

 when it exceeded zo inches. 



If ihe greatcO variations in the heat and prcfTurc of the 

 atmofphere oblervtd in this country did not occafion any 

 variation in regard to the refpcclivc quantities of the two 

 aeriform iluids which compofe it, neither did tlie dilatation 

 or compreflion of the fauie eonunon air, which is in the 

 compound ratio of the variations of the heat and prcHbre, 

 occafion any diBerencc. The niephitic air being the only 

 one of all the aerial fubfianccs which I found incapable of 

 being combined with water, this unalterabilitv fuggcfted to 

 me the id(iii of compofing a permanent inftrument for alcer- 

 taining the greateft or lead dilatation which the atmofpheric 

 air experiences either frcmi one of the two caul'es above indi- 

 cated, or from both of them united. I took a giafs tube of 

 a final! diameter, and, having filled it with water, I then 

 introduced into it a (|uantitv of mephitic air, the fpace occu- 

 pied by which was divided into a hundred equal parts. This 

 fmall tube I put into another larger one containing alfo water 

 to a determinate and conflant elevation, and which was left 

 open to receive the imprenions made bv the variation of the 

 atmofphere, which, prelHng more or lefs on the column of 

 mephitic air, made it extend in proportion, and with I'b much 

 exactnefs and permanency, that at the end offome months 

 it retained the fame dimenfions, which the barometer and 

 thermometer fliowed to be thofe correfponding to the decrees 

 of prelTure and heat. I'his fimpic intlrument enabled n^e to 

 correal, with the greateft precition, the error which fome- 

 tlmt'S arlfes from the dlflerence in the dilatation of the air 

 which I examined, and which mi2;ht readilv occur durinrr 

 the long time necclfary to complete the proof by fulphuret 

 without agitation, obferving the hundredth parts it marked 

 at the beginning and the end of the operation. By fimilar 

 corrci^ions this method, though of long duration, correfponds 

 exaclly with that of fliaking the fulphuret, during which, as 

 the change of dilatation does not take place, the number, 

 conftantly indicated is 21 complete. In a word, during *winr' 

 ter, in fummer, in fpring, and in autumn, in every month 

 and at all hours. I found the air of my country, taken in the 



■. A^* 1.-. . . ^ ' ■' ' ■ - 



open 



