86 Mr Coldstream on the rarer Atmospherical Phenomena 



On the morning of the 17th February, the sky presented, at 

 sunrise, an uncommonly splendid appearance, being covered 

 with long cumuli and cirrostrali, coloured with the richest tints 

 of the spectrum, the green not excepted. The morning after- 

 wards was dull and louring, and in the course of the day 

 heavy nimbi passed in rapid succession from S. W., discharging a 

 few showers of rain. At 4 p. M., while the sky was covered with 

 a thin sheet of cirrostratus and floating sctid-clouds, and when a 

 large nimbus, advancing from the west, was obscuring the sun, 

 then, near the horizon, a halo of an interesting aspect was seen. 

 It consisted of two faintly coloured arcs, one of which had a 

 diameter of about 45°, and the sun for its centre ; the other had 

 a diameter somewhat less, and a point a few degrees above the 

 sun for its centre. The upper arc, therefore, cut the lower in 

 two points, but was not visible within its circumference. The 

 colours of the exterior arc were much fainter than those of the 

 interior. 



The month of March was particularly characterized by the 

 long period of dry weather which then occurred. Only 0.200 

 of an inch of rain fell, and the pressure was very steady and 

 high. 



On the 19th, we were gratified by an unusually fine dis- 

 play of Aurora Borealis, which, as it far surpassed in brillian- 

 cy all similar phenomena seen here for many years past, I 



Particles, has given the following ingenious explanation of the phenomena 

 of white rainbows. 



" In my Theory of Supernumerary Rainbows, {Nat. Phil. ii. 643,) I 

 have observed that the breadth of each bow must be the greater, as the drops 

 which afford it are smaller ; and by considering the coloured figure in which 

 their production is analyzed, it will be obvious, that, if we suppose the co- 

 loured stripes extremely broad, they will coincide in such a manner in one 

 point, as to form a white bow, the red, which projects beyond the rest, be- 

 ing always broadest, so that if all the stripes be supposed to expand, while 

 they preserve their comparative magnitude, the middle of the red may 

 coincide with the middle of the blue ; and it will appear, on calculation, 

 that a white bow will be formed a few degrees within the usual place of 

 the coloured bow, when the drops are about ^ or — of an inch in dia- 

 meter. It is remarkable that, in such cases, the original rainbow is altoge- 

 ther awanting, and probably, for a similar reason, we scarcely ever see a 

 rainbow in a cloud which does not consist of drops so large as to be actu- 

 ally faMmg."— Introduction to Medical Literature, p. 586. 



