VOL. LXXV.J PHILOSOPHICAL TRANSACTIONS. 703 



either our atmosphere must have long ago been deprived of all its electric fluid 

 by being every where surrounded by a boundless conductor, or this fluid must 

 pervade every part of infinite space, and consequently there can be no such thing 

 as a perfect vacuum in the universe. If, on the contrary, the truth of the pre- 

 ceding experiments be admitted, it will follow, that the conducting power of our 

 atmosphere increases only to a certain height, beyond which this power begins 

 to diminish, till at last it entirely vanishes ; but in what part of the upper re- 

 gions of the air these limits are placed, I will not presume to determine. It 

 would not perhaps have been difficult to have applied the results of some of 

 these experiments to the explanation of meteors, which are probably owing to 

 an accumulation of electricity. It is not however my present design to give 

 loose to my imagination. I am sensible, that by indulging it too freely, much 

 harm is done to real knowledge ; and therefore, that one fact in philosophy well 

 ascertained is more to be valued than whole volumes of speculative hypotheses. — 

 Chatham Place, Feb. 12, 1785. 



XV. Experiments and Observations relating to Air and Water. By the Rev. 



Joseph Priestley, LL.D., F.R.S. p. 279. 

 This paper may be consulted in the collection of the author's works, p. 70, 

 vol. 3, anno 178O*. 



XVI. Of the Rotatory Motion of a Body of any Form ivhatever, Revolving, 

 without Restraint, about any Axis passing through its Centre of Gravity. By 

 Mr. John Landen, F. R.S. p. 3 1 1 . 



The substance of this paper may be consulted in Mr. Landen's Mathematical 

 Memoirs, vol.2, p. 83, published anno 1789. 



END OF THE FIFTEENTH VOLUME. 



Erratum.— In p. 37, 1. 28, for 1750, read 1570. 



C. and R. Baldwin, Printers, 

 New-Bridge-Street, London. 



