970 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



bonic acid deposited in combinations in the form of mud. During 

 last Avintcr, when the river was frozen, no oxygen was, of course, 

 absorbed, and the consequence was that there w^ere numerous com- 

 phiints of the impurity of the water. 



Safety-fender for Horse Cars. 

 Mr. John Price exhil)ited a guard to prevent the wheels of cars 

 from injuring persons falling on the track. It was simply a fender 

 made of thick sheet iron placed in front of the wheels, or rather 

 outside of the wheels, at each end of the car, and extending to 

 within a few inches of the track. The exhibitor stated it could 

 be put on a car for thirty dollars. Its weight was 180 pounds. 



Double Rails. 

 Mr. R. Montgomery exhibited specimens of his elevated rail 

 track for wheels having a flange in the middle; also a model of an 

 elevated railway, showing his plan for supporting the rails, which 

 are made like his metallic bridge beams, as shown in the annexed 

 illustration. The pillars which serve as supports can be used as 

 telegraph poles, lamp posts, &c. 



HoLz's Revolving Electrophorus. 

 Dr. Vandcrweyde explained the principle and construction of 

 Holz's new machine for generating electricity manufactured by J. 

 N. and S. Chester, 104 Center street. New York. Two kinds of 

 electricity are known ; one is statical, or that produced by friction, 

 and the other dynamical, or that which is etfected by chemical 

 action, as galvanism, which is known only as a current, while 

 statical electricity may be accumulated. For some time, since the 

 discovery of Galvani, in 1780, statical electricity has been neglected, 

 but of late important discoveries have been made, amongst which 

 was this machine of Holz's, which, in appearance, is not unlike an 

 ordinary plate glass machine ; it does not, however, produce elec- 

 tricity by friction, but by induction. A sheet of paper placed 

 within a short distance of the disc is charged with a small portion 

 of the electric fluid, then the disc is made to revolve, and induc- 

 tion draws from the atmosphere electricity which is collected by 

 metallic points. This machine is worked more easily than the 

 ordinary one in use, but is not intended to supersede it. 



