of Edmburyli, Session 1876-77. 425 
5. Notice of some Decent Atmospheric Phenomena. 
By Professor Tait. 
(Read 5th June 1876.) 
On March 25th, at 8.15 p.m., very few clouds being visible, 
there appeared a luminous arch in a vertical plane, passing true 
E. and W. nearly through the zenith. This arch, to my surprise, 
gave everywhere a continuous spectrum. I had, in company with 
Professor Dewar, observed a phenomenon similar in all respects 
(continuous spectrum included) some seven years before, and was 
then inclined to fancy the arch a mere low-lying cloud, and its 
light due to the town lamps. But on the present occasion, after 
about forty minutes, when the arch bad disappeared, an aurora, 
showing unmistakably in the spectroscope the characteristic green 
band, appeared near the magnetic north. And a few days after- 
wards there appeared in the Edinburgh newspapers several descrip- 
tions of a similar arch seen at almost exactly the same time at 
Colinton, &c., and even so far off as Dunfermline. Most of the 
observers remarked the entire absence of coruscations as a proof 
that the phenomenon could not be auroral. The facts stated seem, 
however, to leave no doubt that it was connected with the aurora, 
and the peculiarity of its spectrum remains to be explained. 
On May 10th, the sky cloudless but hazy, from 11 a.m. till 
nearly 3 p.m., I observed a magnificent halo of 22°, with its 
tangent arcs forming almost a complete external oval. Various 
observers in different parts of Scotland seem to have noticed the 
halo itself from 2 p.m. till sunset, but none have mentioned the 
external oval, which at midday was brighter than the halo. 
As showing that the ice-crystals in the air were very widely 
spread about this period, I append an extract from a letter I have 
just received from Mr Aitken of Darroch. 
“ I have sent you a sketch I made of an atmospheric pheno- 
menon which I saw at Cadenabbia, Lago di Como^ on Sunday the 
30th April, and hope it may be useful to you to-night. 
“ The weather during the previous day was very bad ; it rained 
heavily all day. The wind was northerly, and the temperature 
low. In the evening there was a thunderstorm, accompanied by a 
good deal of lightning. Sunday morning broke clear and tine ; 
