British Association for the Advancement 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 curious 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 
not 
which many members had urged him to bring fectieation DU 
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 G 
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 
new 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. Mr. 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- 
steps ii cqudiaeaiieacdl 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 
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