British Association for the Adva7icement of Science. 37 



nied by an increase of temperature of about 6.25°, or about one 

 degree for sixteen yards ; the result of the observations made in 

 France giving one degree for about each fifteen yards of descent. 



Magnetism. — Mr. Fox drew attention to the advantages con- 

 ferred on this branch of science by Lieut. Burns. He exhibited 

 charts drawn by him, in which the dip and variation were laid 

 down, with extreme accuracy, in several parts of the world. He 

 gave a cmious instance of the value of a knowledge on these sub- 

 jects ; as the ship drew near a promontory of the Cape de Verd 

 Islands, the action of the rocks became perceptible to this accurate 

 observer, who was thereby warned of the neighborhood of land. 

 He also was frequently able to guess at the nature of the rocks at 

 the bottom of the sea from similar indications. — Major Sabine 

 explained, that the Report now laid before the Section, related 

 exclusively to observations made in Great Britain ; hence he had 

 not alluded to any of these subjects, nor to other general results 

 which many members had urged him to bring forward. — Prof. 

 Phillips said, that the dip changed frequently in the course of 

 a day ; in some observations which he had made at several sta- 

 tions between Ryde and York, he had found the dip to vary so 

 much as six or seven minutes in the course of the day. 



Diamond. — Sir D. Brewster now read a notice of a new struc- 

 ture in the diamond. 



Sir David said, that having communicated to the Geological 

 Society an account of certain peculiarities in the structure of the 

 diamond, which confirm the theory of its vegetable origin, he 

 was desirous of submitting to the consideration of this Section a 

 ncAV structure which he had recently detected in that gem, and 

 which indirectly supported the same views. In consequence of 

 the diamond having been used as the fittest substance for form- 

 ing single microscopes of high power and small spherical aberra- 

 tion, the attention of opticians has been drawn to the imperfec- 

 tions of its structure. INIr. Pritchard, who first succeeded in ex- 

 ecuting lenses of diamond, put into the hands of Sir David for 

 examination, a plano-convex lens about the 30th of an inch in 

 diameter, which he had found unfit for the purposes of a micro- 

 scope in consequence of its giving double images of minute ob- 

 jects. As Sir David had previously shown, that almost all dia- 

 monds possessed an imperfect doubly refracting structure, as if 

 they had been aggregated by irregular forces, compressed or 



