798 



PHILOSOPHICAL TRANSACTIONS. 



[anno 1800. 



The refractions on the 2d and 3d arcs, I consider as most accurate, on account 

 of the great distance between the stations ; and also as more to be depended on, 

 from the circumstance of the ray generally passing 300 feet above the ground. 

 The 4th arc affords another instance of the refraction varying at one station, and 

 remaining constant at the other. This was owing to the intervention of some par- 

 tial stratum of air, nearer to Epwell than Broadway Beacon. The refractions, de- 

 duced from these contemporary observations are inconclusive. The mean refractions, 

 neglecting the 4th arc, brought under one point of view, will be as follows. 



Arcs. 



Mean height 



of ray above 



the sea. 



Feet. 



734 



774 



854 



Refraction. 

 Propl. pt. 



Barom. Therm. 



in. pts. 

 29.5 

 2().2 



28.8 



57.8 

 67.8 

 58.1 



] t White Horse Hill and Whiteham 



bury Hill and Bril, 5 first refracs. - 



3. Arbury Hill and Wendover - 



If the air had been in a quiescent state, previous to and also at the times when 

 these observations were made, it might be expected that the differences of altitudes 

 in the stations would be obtained, tolerably near the truth, barometrically. The 

 remarks in the tables appertaining to the 1st and 2d arcs, shew that such opportu- 

 nities offered ; but those which belong to the 3d, prove the wind to have been 

 fresh ; and, as the space between the stations which constitute the extremities of 

 that arc is 34 miles, nearly, it is not to be expected that a true result should be 

 obtained. The differences of altitudes of the stations constituting the extremities 

 of the first 2 arcs, obtained by means of the observed angles of elevation and de- 

 pression, as well as from the heights of the mercury in the barometer, will be, 



Arcs. Obs. Ang. Barom. Diff. 



1 3J7 282 35 



2 60 15 45 



The little that has been done on this subject, points out the necessity of doing 

 more ; it therefore remains to observe, that I shall lose no opportunity of employ- 

 ing the apparatus committed to my charge in the best and most diligent manner, 

 both as relating to matters of refraction, and to all others connected with the 

 Trigonometrical Survey. ^_ 



Note referred to at page 746 of this Volume. 

 It was intended to subjoin to Mr. Volta's description of his electrical pile or battery, an account of 

 Mr. Davy's discoveries, which constitute a new aera in chemistry, relative to the decomposition of the 

 alkalies and their metallization, by means of the above-mentioned apparatus. But on further reflection 

 it appeared, that even a summary statement of those brilliant discoveries would far exceed the limits of 

 a note, and indeed was scarcely necessary, from the circumstance of Mr. Davy's experiments having 

 been so lately communicated to the public, in the Philos. Trans, (see Bakerian lecture, 1808) which, 

 from the commencement of the present century, where our labours terminate, are, or will be, it ii 

 presumed, in the possession of the Subscribers to this Abridgment. 



END OF VOL. XC OF THE ORIGINAL, AND OF THIS ABRIDGMENT. 



INDEX. 



