1884.] 
Observations on Twilight. 
451 
points lower and lower down until it reaches the sea-coast 
creating bars at the mouths of rivers in its course, and 
changing the hydrography of harbours — as it has done with 
the .Bay ot San Francisco by its silt. 
The hills behind, torn up and washed by the gold miner, 
are abandoned as desolate and irredeemable ; and the costly 
canals, constructed with peculiar conveniences for mining 
purposes, eventually fall into disuse from being too expensivS 
to maintain or alter for general agricultural uses. 
130, Fenchurch Street, London, E.C., 
May 14, 1884. 
II. OBSERVATIONS ON TWILIGHT. 
SffikT E must first here complain of the want of a word 
which exactly covers our meaning. We might 
have said “ Observations on Dawn,” but this term 
is by a general convention limited to the glow in the sky 
which precedes sunrise, excluding that which follows 
sunset. 
The remarkable atmospheric phenomena which were wit- 
nessed in most parts of the world during the last winter 
have, as our readers know full well, given scope for much 
discussion ; but, oddly enough, for some time no one raised 
the necessary preliminary question touching the normal 
colouration of the sky at morn and eve. At last the thought 
occurred, both to theorists and observers, that without a 
precise knowledge of this normal colouration all inquiry as 
to the abnormal display of the last season was in want of a 
firm basis. Were the aerial pageants described by “ Our 
Special Correspondents ” merely an intensification of what 
has always been witnessed under cloudless skies, or were 
they something novel and totally different ? 
It might indeed seem strange that, in a scientific age like 
the present, hundreds of observers were not at once prepared 
with an answer ; but the observations required can be made 
to advantage only under skies fairly clear from cloud and 
