PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 855 



allowed to find its level by opening a valve when setting the 

 instrument. The top of the funnel is closed by a septum, consist- 

 ing of Wedgewood's ware, secured in its place by common sealing 

 wax. The other end of the battery Avire is connected with the 

 instrument. When the apparatus is placed in a mixture of com- 

 mon air and marsh gas, tiie latter passes through the septum more 

 rapidly than the air within the apparatus can escape, according to 

 the well known law that gases diffuse into one another in the 

 inverse proportion of the square roots of their specific gravities. 

 The accumulation of marsh gas within the septum, therefore, 

 . increases the pressure on the mercury in the funnel, and thus raises 

 it in the leg of the U-tube, until it touches the platinum point, 

 when electric communication is established, and the alarm is given 

 by means of a bell having wheel work for moving the clapper, 

 which is set in motion the instant the circuit is closed. Mr. Ansell 

 has found his instrument gives warning in four seconds, if the mix- 

 ture of gas is still below the point of explosion, but, hy adjusting 

 the point so that there is not more than the thickness of a shilling 

 between it and the mercury, a dangerous irruption of gas may 

 make itself known in two seconds. 



Although this apparatus is said to he efficient, it has not yet 

 been generally adopted. Some superintendents of mines assert 

 that the flame of the safety lamp becomes dull in the presence of 

 marsh gas, 'and thus warns them of danger, but the numerous acci- 

 dents in mines prove that such warning is disregarded. 



A French savant has sug-o-ested the use of Ruhmkorff's induction 

 apparatus for illuminating the galleries of mines, by which ordi- 

 nary lights would be dispensed with, and all danger of explosion 

 avoided. 



THE HARRISON BOILER. 



Mr. J. Burrows Hyde read the following paper descriptive of 

 the cast iron steam-boiler invented by Joseph Harrison, Jr., Esq., 

 of Philadelphia : 



This boiler is constructed by uniting or building up a requisite 

 number of cast iron hollow spheres, provided with hollow necks 

 or unions, made to fit by close steam tight joints, and so connected 

 as to form a rectangular slab or section of any convenient capa- 

 city. 



The globes are all of uniform dimen^ons, being eight inches 

 external diameter, and three-eights inch thick, the union necks 



