382 ASSINNIBOINE AND SASKATCHEWAN EXPEDITION. 



solemnity and charm to the still night, which must 

 ever remain one of its most delightful characteristics in 

 these regions. 



THE TWILIGHT BOW. 



One of the most beautiful celestial phenomena visible 

 after sunset and before sunrise from the north-western 

 prairies is the Twilight Bow. The extraordinary clear- 

 ness of the nights during summer in this region oners a 

 very favourable opportunity for witnessing the delicate 

 colouring which is communicated to the lower atmosphere 

 by the reflected light from the upper illuminated portions. 

 As the appearance of the Twilight Bow is dependent upon 

 the serenity of the atmosphere to a great degree, its oc- 

 currence is not frequently observed or recorded in this 

 country. 



The Twilight Bow, and the causes which produce it, are 

 thus described by M. Bravais * : — " Immediately after the 

 setting of the sun, the curve which forms the separation 

 between the atmospheric zone directly illuminated by the 

 sun, and that which is only illuminated secondarily, or by 

 reflection, receives the name of the Crepuscular curve or 

 Twilight Bow. Some time after sunset, this bow, in tra- 

 versing the heavens from east to west, passes the zenith ; 

 this epoch forms the end of civil twilight, and is the 

 moment when planets and stars of the first magnitude 

 begin to be visible. The eastern half of the heavens 

 being then removed beyond solar illumination, night 

 commences to all persons in apartments whose windows 

 open to the east. Still later the Twilight Bow itself dis- 



* Annitaire Meteorologique de la France for 1850 ; quoted by L. W 

 Meek in the Smithsonian Eeport for 1856. 



