CHAPTER IV. 



THE RESULTANT PRESSURE RECORDER. 



As preliminary to obtaining the data mentioned at tlie close of the last 

 chapter, it is desirable to determine experimentally the direction of pressm*e of 

 the air, (since the air is not an ideal fluid such as the theory contemplates,) on an 

 inclined plane, and to investigate the assumption made by Xewton that the 

 pressure on the plane varies as the square of the sine of its inclination. 



The second instrument constructed was, then, for the purpose of obtaining 

 graphically, the direction of the total resultant pressure on an inclined plane 

 (in practice a square plane) and roughly measuring its amount.* For this reason 

 it will be called here the BesuUant Pressure Recorder. 



DESCRIPTION^. 



Plate V contains drawings of the instrument. Upon a base-board, BB', is a 

 standard, E, carrying an arm, AA', hung symmetrically in gimbal joints. On 

 the outer end of the arm a one-foot-square plane (called here the wind plane) is 

 fastened with a clamp, and a graduated circle assists in setting the plane at 

 different angles of inclination to the horizon. The extremity of the inner end of 

 the arm carries a pencil, P, which registers on the surface of a vei'tical plane, which 

 is in practice a sheet of diagram paper clamped on the surface FF' of an upright 

 circular board fixed by a standard to the base-board BB'. The pencil-holder H 

 fits closely into a ring at the center of a system of four equal radial springs attached 

 to a circular frame, ]MM', projecting immediately in front of the registering 

 board and concentric with it. This frame MM' is connected by supports to a 

 close-fitting ring, which closes around the registering board and serves as a holder 

 for the diagram sheets which are, as stated, clamped on the face FF' of the cir- 

 cular board. The radial-spring system and its frame may be rotated about the 

 registering board, so that the diagram sheet may be rotated in its own plane. 

 The inner or recording end of the arm is weighted so as exactly to counterpoise 

 the outer end carrying the wind plane. Hence this plane is virtually weightless. 



* Observations of the pressure on inclined planes have been maile by previous experimenters, the flrst being 

 by Hutton in the summer of 17S8, just 100 years before those about to be recorded. But in the experiments of 

 Hutton, as well as in most of the later ones, the horizontal component of the pressure on the inclined plane ho.s 

 been the subject of measurement, while the apparatus about to ba descriljed affords a measurement of the total 

 normal pressure on the plane. 



