VOL. LXXVIII.] PHILOSOPHICAL TRANSACTIONS. 407 



heat of the top and bottom, while that in a corresponding situation in the well 

 at Sheerness gave it colder than either top or bottom, he attributes to the follow- 

 ing circumstance. Over the well at Sheerness a machine is erected, which raises 

 the water by means of an horizontal windmill, working an endless chain. This 

 chain, consisting of jointed double bars, with a number of buckets fixed at certain 

 distances from each other, continually descending into, and ascending out of the 

 water, to an elevation of 8 or 9 feet above the top of the well, may be supposed 

 to reduce the water as far as it reaches to the mean temperature of the air above ; 

 and thus he found it ; for 51° had been the mean temperature of the air near the 

 sea-shore for several days before. At the bottom of the well, near to which the 

 chain never descends, he found the temperature 56° ; above 7*^ warmer than that 

 at Dover well. 



The water at the bottom of these wells is, he presumes, too deep beneath the 

 surface of the earth ever to be affected by the temperature of the atmosphere ; for 

 if the heat of the summer could have had any influence on either of them, that 

 at Dover must have been most considerably affected by it, especially in the month 

 of September ; and the air was something warmer when the experiment was made 

 at Dover than at Sheerness. From the nature of the different kinds of strata in 

 which these wells are dug, had they been in all other circumstances the same, one 

 might reasonably expect to find the warmer spring in the chalk, and the colder in 

 the clay ; but here the reverse is seen, without any apparent local cause, except 

 the different elevations of the springs in respect to the level of the sea. 



IK. Observations on the Manner in which Glass is Charged with the Electric 

 Fluid, and Discharged. By Edw. Whilaler Gray* M. D., F. R. S. p. 121. 



Dr. Franklin, in various parts of the first volume of his experiments and ob- 

 servations, asserts. That the natural quantity of electric fluid in glass cannot be 

 increased or decreased ; and that it is impossible to add any to one surface of a 

 plate or jar, unless an equal quantity be, at the same time, given out from the 

 other surface.-]- This error has been adopted by succeeding electricians; among 

 others, by the late Mr. Henly, who in one of his last papers, printed in the 

 Philos. Trans, for the year 1777? has the following words: " According to Dr. 

 Franklin's theory, the same quantity of the electric matter which is thrown upon 

 one of the surfaces of glass, in the operation of charging it, is at the same time 

 repelled or driven out from the other surface ; and thus one of the surfaces be- 

 comes charged plus, the other minus ; and that this is really the case is, I think, 



* Dr. Gray died in Jan. 1807, being 59 years of age. At the time of his death he was senior secre- 

 tary of the R. s. and keeper of the department of natural history and antiquities at the British 

 Museum. 



t " The quantity proportioned to glass it strongly and obstinately retains, and will have neither 

 more nor less." Experiments and Observations, vol. i. p. 26. See also p. 75, 81, et alibi. 



