VOL. XXV.] PHILOSOPHICAL TRANSACTIONS. 355 



while, it grew by degrees weaker and weaker, so that in about a quarter of an 

 hour after I first saw it, the top-part (a, b, d, in fig. 3, pi. 10,) was scarcely 

 visible. But the lower part remained vivid much longer, but yet grew by 

 degrees shorter and shorter. I saw the remains of the lower half (bdef) a 

 full hour after sun-set ; and should perhaps have seen it longer, had the 

 horizon been open ; instead of which it was often in my walk obstructed by 

 trees. The whole atmosphere seemed hazy, and full of vapours, especially 

 towards the sun-set. The moon and stars were bearded at that time, and 

 succeeded with a halo about the moon afterwards. Which disposition of the 

 air was probably the cause of the phenomenon. But the pyramid was 

 doubtless imprinted on the far distant vapours of the atmosphere ; it being 

 manifestly farther off, or laying beyond some small thin clouds (clcl) that 

 intercepted it, and in those parts covered and hid it. I do not remember I 

 ever saw any thing like it, except the white pyramidal glade, which is now 

 called the aurora borealis. And it being, except in colour and length, so like 

 that, I have thought it may perchance in some measure conduce to the solu- 

 tion of that odd phenomenon, the aurora borealis. 



j4h Experiment^ confirming the Producdon of Light, by the Effluvia of one Glass 

 falling on another in Motion, liy Mr. Fr. Hauhsbee, F. R. S. N^ 3 ] 0, p. 23 13. 



Having observed that the effluvia of glass were capable of exhibiting a phe- 

 nomenon falling on an exhausted glass in motion, as if rubbed by a visible 

 solid body; I thought a farther confirmation of the same, would not be unac- 

 ceptable. For that purpose I took a large receiver, as shown by aaaa, fig. 

 4, pi. 10. Within the body of which I fixed another, as b b b b, their axes 

 lying parallel to the horizon, and fixed one within the other at cc. The 

 outer surface of the inner glass was at least an inch distant from the inner 

 surface of the outer one; and they were turned by two large wheels, whose 

 bands communicated with the small wheels dddd, fixed on their axis. The 

 inner glass was first exhausted of its air ; then being fixed, as abovementioned, 

 I moved only that wheel, which gave motion to the large glass; thinking 

 that when the effluvia of that glass, by the application of my hand on it, 

 should reach the other, though it was at rest, it would nevertheless be affected 

 by it, and give a light; which happened accordingly, spreading itself in flying 

 branches all over. Then turning the other wheel, the light became more con- 

 siderable, and the greatest that has yet been produced in any experiment made 

 on this subject. I caused both the great wheels to give motion to the glasses 

 one and the same way, with as equal a velocity as they could ; yet I did not 

 discover but the light was then as strong, and as lasting, as when their motions 



z z 2 



