CHAPTER IX. 



THE ROLLING CARRIAGE. 



The Boiling Carriage was constructed for the purpose of determining the 

 pressure of the air on a plane moving normal to its direction of advance.* What- 

 ever be the importance of this subject to aerodynamics or engineering, we are 

 here interested in it only in its direct bearing on the aerodromic problem, and 

 carry these observations only as far as this special object demands. Before this 

 instrument was constructed, a few results had already been obtained with the 

 Resultant Pressure Recorder (chapter IV), but additional observations were desired 

 with an instrument that would be susceptible of greater precision. The state- 

 ment has frequently been made that the law that the pressure is proportional to 

 the square of the velocity fails for low velocities as well as for very high ones. 

 As it appears to me that this conclusion was probably based on imperfect instru- 

 mental conditions due to the relatively excessive influence of the friction of the 

 apparatus at low velocities, particular pains were taken in the present experi- 

 ments to get as frictionless an action as possible. Plates IX and X contain 

 drawings in elevation and plan of the apparatus devised for this purpose. 



A metal carriage 8J inches long is suspended on a set of delicately con- 

 structed brass wheels 5 inches in diameter, which roll on planed ways. Friction 

 wheels bearing against the sides and bottom of the planed ways serve as guides 

 to keep the carriage on its track. Cushions of rubber at each end break the 

 force of any end-thrust. Through the center of this carriage passes a hollow 

 brass rod 27h inches long, on the forward end of which is set the wind-plane by 

 means of a socket at its center. On the other end is attached a spiral spring, 

 which is also fastened by a hook to the roar of the carriage-track in a manner 

 illustrated in the drawing. The rod is of such length that the wind-plane may 

 ho removed from the disturbing influence on the air of the mass of the registering 

 apparatus, and the center of gravity of wind-plane and rod falls under the center 

 of gravity of the carriage. The pressure of the wind on the wind-plane is bal- 



*Thcse measurements of pressure on the normal plane an- not presented as new. Thej were made a- a 

 in i essary part of an experimental investigation which aimed to take nothing "ii trust, or on authority however 

 respectable, « it In mi verification. They are in one sense supplementary to th< others, and although made early 

 in the course of the investigations pre-ented in this memoir, are here placed last, so as not to interrupt the 

 presentation of the newer experiments, which are relate.! to each other by a consecutive developm* tit. 



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