Hennessey— 0?i Phenomena in Optical Meteorology. 247 
was easily verified by sighting it from a window. It lasted only for 
a few minutes, and when it disappeared I noticed a reflection of the 
rainbow on the water. On this account we might be led to think 
that the luminous vertical column was due to the refraction and dis- 
persion of light reflected from the top of the waves. This does not 
perfectly explain the position of the column as tangent to the bow. As 
the reflecting surface furnished by the water was more unbroken and 
wider at this side, the tangential beam of light might be due to re- 
flexion of the lower nearly straight and extremely brilliant limb of 
the bow. All through the primary bow was remarkably brilliant, 
which was due, no doubt, to the largeness of the rain drops falling 
during the thunder showers. 
I^fovember 21st, about 10 p.m., the sky cleared after rain and 
high wind, and the air had become calm. A thin film of cloud covered 
the moon's disc, which was nine days old. A small corona (about 
6** in diameter probably) surrounded the disc, exhibiting brilliant 
prismatic colours, while outside this was another corona, much paler, 
and not complete. 
December 9th, at 2*30 p. m., I noticed a halo around the sun. 
Ey rough estimation, it appeared to have a radius of about 40°, which 
would bring it into a class of halos not most commonly observed. It 
was very sharply defined with faint prismatic colours. At one time it 
appeared, for a very brief interval, to have three points of maximum 
brightness — namely, at two points on the horizontal diameter, and at 
the point vertically over the sun. It disappeared about three o'clock, 
when the sky became covered with many light scattered clouds. 
The mountains forming the background were covered with fresh 
snow, some of which had fallen during the preceding day or night, 
and the thin film of cloud in which the halo was visible was mani- 
festly higher, and therefore composed of snow crystals, as might be 
- expected from the established theory of halos.'^' 
On December 19, I observed a faint and imperfect halo around 
the sun, at half-past two p. m. ; its apparent radius much less than that 
of the halo just described ; and it was probably about 23°, or that most 
usually observed. A bank of clouds was rising from the S. W., and 
the halo soon disappeared. 
XXIX. — On the Actio]!^ of Heat upon" Solutions of Hydrated Salts. 
By Charles K. C. Tichboene, F. C. S., M. R. I. A. (With PL XIX., 
Science.) 
[Read January 8, 1872.] 
When considering the dissociative action of heat upon water of hydra- 
tion, some evidence, was naturally sought amongst those salts which 
present a change of colour when passing from the dry to the hydrated 
* An elaborate discussion of the theory of halos is given in the well-known 
memoir by M. Bravais, Journal de 1' Ecole Polytechnique, 1817. 
