L. MECHANICS AND ENGINEERING. 533 



and the train was stopped in ten seconds, passing over 318 

 feet of track. 



Other equally satisfactory trials were made by applying 

 the brake and stopping the train while the engine was still 

 working, and by testing its automatic action by stopping 

 the train whenever this meets with an obstruction of the 

 track. 



An iron rod connects with each brake, extending down to 

 within four inches of the rail, and to this is attached a cross- 

 bar. This device will apply the brake, whenever it comes in 

 contact with an obstruction on the rail, or in case of the 

 breaking of the wheels. 



In the experimental trial referred to, the efficiency of this 

 attachment was tried by means of a shovel while the train 

 was traveling at moderate speed, and the train was speedily 

 brought to rest. 



The experimental tests were uniformly successful, and 

 demonstrated most clearly the immense superiority of the 

 automatic system over the old plan of braking by hand. 



The official report of the Committee, which is at present in 

 course of preparation, will doubtless do much toward draw- 

 ing increased attention to the automatic system. 



THE MECHANICAL PEINCIPLES OF FLYING. 



TJ Aeronaute^ a journal devoted to the interests of aerial 

 naviofation, contains a valuable contribution to our knowl- 

 edge of the mechanical principles of the flight of birds, in an 

 article by M. A. Penaud. The elaborate experiments of Thi- 

 bault on the resistances opposed by the air to motions of 

 thin plates of metal form the basis of the mathematical stud- 

 ies of Penaud, as well as of those by Louvrie, published in 

 1868. Thibault's experiments showed that in moving a 

 plane square surface the resistance normal to the surface re- 

 mains very nearly constant so long as the angle between the 

 normal and the direction of motion (the angle of incidence) 

 is included between 90 and 45 ; it then diminishes pro- 

 gressively to 20, from which point up to of incidence it 

 is sensibly proportional to the sine of the angle. M. Penaud 

 now demonstrates, first, that a bird sailing in the air falls as 

 slowly as possible when he employs for his horizontal move- 

 ment one fourth of the work of the fall; second, a bird in 



