bn one or both fides of the inftrument. Note, for greater 

 accuracy this procefs may be repeated two or three times, and 

 Ihould the feveral obfervations vary, a mean of all may be 

 taken. 



Remarks on the ConJiruB'ton and U/e of the Injirument. 



REMARK I. 



On the conftrudlion of this inflrument very little need be faid, 

 as it is {o obvioufly fimple and eafily to be apprehended. It 

 may not, however, be ufelefs to remark that the author has, 

 by the ufe of a variety of thefe inftruments for a number of years, 

 fatisfied himfelf and feveral ingenious and philofophical friends, 

 that the permeability of cork to air, and at the fame time its 

 refiftance to the paflage of mercury, is mofl fatisfadorily afcer- 

 tained, a point which cannot be too ftrongly infilled on in this 

 bufinefs, as the fuccefs of the experiment depends entirely on 

 the truth of this fad ; in making the inftrument, great care 

 fliould be taken to provide found, fmooth, and fpungy corks, 

 to fit the round of both tube and cylinder very accurately, and 

 to be careful not to force them into their places too tight, which 

 would not only endanger breaking the tube, but alfo make the 

 rife and fall of the mercury, on inverting the barometer, too 

 tedious. It will be alfo requifite, in filling the ciftern, to 

 obferve accurately the quantity of mercury that will fuffice to 

 keep the end of the tube covered in all pofitions, and at the fame 



time 



