Dr Brewster on a new Cleavage in Calcareous Spar. 311 



2. The yearly Variation from Minimum in February to 

 Maximum in July is also muck greater in Christiania 

 than in Leith. 



Christ. Leith. 



Mean of Febr. = 16°.224 40°.621 

 July, == 61 .690 60 .361 



Difference, = 45°. 466 19°740 



The cause of these small variations in the climate of Leith 

 I suppose to be the mist from the sea, and the unclear sky, 

 which tempers the cold of the night and of the winter, and 

 absorbs the heat of the rays of the sun during the day and 

 in summer. 



Christiania, 17th April 1828. 



Art. XX. — On a New Cleavage in Calcareous Spar, with a 

 notice of a method of detecting Secondary Cleavages in Mine- 

 rals. By David Brewster, LL. D. F. R. S. Lond. and 

 Sec. R. S. Edin. 



In different papers printed in the Philosophical Transactions, 

 (1815, p. 270,) in the Edinburgh Transactions (vol. viii. p. 

 165,) and in the Transactions of the Geological Society (vol. 

 v.) I have had occasion to direct the attention of philosophers 

 to the optical and mineralogical properties of a very interest- 

 ing, though a very common structure in calcareous spar. This 

 structure displays itself in the appearance of one or more re- 

 flecting planes parallel to the edges which contain the obtuse 

 angle, and bisecting the acute angles of the rhomboidal faces. 

 These reflecting planes were regarded by Huygens, (who 

 first observed them,) Brougham, Mohs, and other philoso- 

 phers, as fissures in the crystal, and on that supposition they 

 endeavoured to explain the curious and beautiful optical phe- 

 nomena to which they gave rise. From a minute examination, 

 however, of a great number of specimens, I succeeded in de- 

 termining that these reflecting faces were the faces of thin 



