[ i6 9 ] 



Thefe three accidental errors, 

 though fmall in themfelves, yet may 

 amount to fix or more fub-di virions. 

 So that all the errors already enu- 

 merated may amount, if they were 

 all committed together, to 260 fub- 

 divifions. 



Befides all the above-mentioned 

 fources of errors that may be com- 

 mitted, either by indifferent inflru- 

 ments, or by want of proper atten- 

 tion, there is one which had always 

 vexed me in former times, and 

 which feemed to me, as well as to 

 many others, almoft incorrigible. 

 This is the inconftancy in the qua- 

 lity of the nitrous air, which is 

 found fometimes much ftronger or 

 weaker than at other times, though 

 the fame method of producing it 

 has been obferved. 



Of 



