392 



MISCELLANEOUS. 



moon, divided and extinguished them. In the aBa Erudttormjiy 

 printed at Leipsic*, it will be seen that, on the 4th of March,. 

 1728, an arc, formed by the concurrence of several clouds, 

 was observed at Germendorf : its projedtion, N. E. and S. W. 

 led to a supposition that it originated in the S. E. whence the 

 wind then blew. 



'* May I be permitted to hazard another conje6lure, in an 

 age when physical novelties are rated at so high a price. I be- 

 lieve that the above-mentioned arc was a true iris, occasioned 

 by the refie6lion of some of the stars which were setting. The 

 demonstration is clear : it being granted that the atmosphere 

 was charged with a multitude of atoms, a,nd terrestrial vapours, 

 these would, with the cold of the night, be gradually con- 

 densed, and would descend, according as their gravity should 

 exceed that of the air in which they swam, as happens in the 

 case of rain. As our situation was between these descending 

 exhalations and the above-mentioned stars, the former of which 

 were in front of us, and the latter at our side, it follows that 

 the centre of the one, or of the other, would, according as we 

 varied our position, coincide wdth our optical axes. This hy- 

 pothesis being admitted, all the luminous rays with which the ' 

 constellation invested the vapours, would fall on the eye of the 

 spe6lator at the same angle, equal to that of its incidence. The 

 objedls which are seen at an identical angle, appear to be at 

 an equal distance ; but this equal distance could not have been 

 verified, unless the atoms which divided the valley had repre- 

 sented an arc, according to the principles of optics and geo- 



* December, 1730. 



metry : 



