involved in the Construction of Artillery. 



165 



bottom or lowest point, in place of the metal being thrown in at the open top, 

 with a fall at first of several feet, as is now the common practice, by which much 

 air and scoria are carried down and mixed with the metal, some of which never 

 rises up again, or escapes as " air bubbles." But the great value of increased 

 head of metal, in adding to the density of castings, and so also to their strength, 

 appears so little generally known, or, if recognised, is so seldom attempted to 

 be practised, to any considerable extent, i. e. depth of head, that I am induced 

 here to repeat the Tables xii. and xiii. from pp. 304 and 305 of my Second 

 Report on Iron, Transactions of the British Association for 1840, in which the 

 results are given of some extended and careful experiments made by me, to 

 ascertain the relation between the head of fluid pressure and the specific 

 gravity of the casting. 



55. My experiments were made upon cylindrical shaftsof cast-iron, cast ver- 

 tically, in dry sand moulds, and under heads gradually increasing up to four- 

 teen feet in depth ; and all poured from " gaits" at the bottom. 



Table I. 



Showing the increase of Density in Castings of large size, due to their Solidification under a 

 head of Metal, varying from two to fourteen feet in depth. 



These experiments show an increase of density due to 14 feet head about 

 equal to a pressure of 44-8 lbs. per square inch ou the casting, from 6-9551, to 

 7-1035 for Scotch cast-iron. 



56. In the following Table, No. ii., the decrease of specific gravity following 

 increase of bulk is obtained. 



VOL. XXIII. Z 



