282 Estimate uf the Force of Explodv7i of Coal Gas. 



but this excess will be compensated, when the velocity is after- 

 wards communicated to the surrounding atmosphere. With 

 this velocity, the centre of inertia of each elementary pyramid 

 of the sphere will advance from the distance | X 1 5 to J x 33 .35 feet, 

 through 13f feet, in -^ of a second : and at any greater dis- 

 tance d, the velocity of the impulse will be reduced from 2000 



to 2000 X , or , its duration being always -^\^ of a 



d d 



second. Thus the velocity of a very high wind being 60 feet in 

 a second, the impulse would retain this force at the distance of 

 833 feet : and in order to determine at what distance it would 

 overset a wall 9 feet high, and 1 J thick, we must first find the 

 height through which the centre of oscillation of the wall, at j 

 of its height, must ascend, in order to be immediately over the 



point of support, that is ,J (36 -f 1) — 6 = — of a foot: and 



the velocity corresponding to this height would be generated 



bv the force of gravity in / = of a second : and 



^ ^ -^ V J93 13.9 



in order to be generated in -^^^^ it requires a force 10.43 times 

 as great, or equal to the pressure of a column of brick 15.64 

 feet high ; that is, 15.64 x 125 = 1955 pounds for every square 

 foot; which is equivalent to the pressure occasioned by a velocity 

 of 966 feet in a second, and answers to a distance of 52 feet. 



Art. VIII. On the Crystalline Forms of Artificial Salts. 

 By Mr. Levy. Communicated by the Author. 



The relation between the chemical composition of a sub- 

 stance, and its crystalline form, has not yet been ascertained; 

 and it is only from a comparative examination of the exact 

 analyses and forms of a great many simple and compound 

 bodies, that it may be expected to be deduced. The data 

 furnished by Miaeralogy are not sufficient to discover it ; be- 

 cause not only there are too few simple compounds found 

 crystallized, but also because those which are met with have 



