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thing to know whether the air paffes into the cyU" with a 

 velocity variable according to its denfity, and in what ratio. 

 The time of the air's paflage cannot be difcovered by the mo- 

 tion of the merc>' in the gage •, for when the air is much ra- 

 refied, this motion during one flroke, is fo little as to be im- 

 perceptible, and when it is greater, the mere*" vibrates fo much 

 in the tube, and undulates fo long, after the air is pafTed into 

 the barrel, that the term of its pafllige cannot be difcovered: 

 it mufl: be found by reafoning on phyfical principles ; and the 

 enquiry is to be profecuted on thefe data or alTumptions, that 

 the meafures of the recr, duct and cyh" are invariable quanti- 

 ties ; and that in the cyl^ there is a perfedl vacuum, into which 

 the air in the rec' is to expand itfelf, after the pifton is raifed 

 to the top. 



The conftrudlion of this machine was primarily undertaken 

 with a view to difcover whether the Aurora Borealis is an 

 eledlrical phsenomenon ; whether we could in large exhaufted 

 veflels exhibit its genuine appearance ; what muft be the ra- 

 rity of the air in which it is vifible, and the heights in the 

 atmofphere, wherein it exifts, and within which it is (if at all) 

 confined, &c. Thefe enquiries however I did not profecute ; 

 yet the reader will not I hope be difpleafed at my extend- 

 ing this long paper ftill farther by an account of the follow- 

 ing experiments relative to them, though they are inconclufive ; 



efpecially. 



