THE RESULTANT PRESSURE RECORDER. 25 



would otherwise be valuable. Occasional experiments have been made since the 

 time of Newton to ascertain the ratio of the pressure upon a plane inclined at 

 various angles to that upon a normal plane, but the published results exhibil 

 extremely wide discordance, and a series of experiments upon this problem 

 seemed, therefore, to be necessary before taking up some newer lines of inquiry. 

 The apparatus with which the present experiments were made, was designed 

 to give approximations to the quantitative pressures, rather than as an instru- 

 ment of precision, and its results are not expected to afford a very accurate 

 determination of the law according to which the pressure varies with the angle 

 of inclination of the surface to the current, but incidentally the experiments 

 furnish data for discriminating between the conflicting figures and formuke that 

 now comprise the literature of the subject. We may remark that they incident- 

 ally show that the effect of the air friction is wholly insensible in such experi- 

 ments as these; but the principal deduction from them is that the sustaining 

 pressure of the air on a plane 1 foot square, moving at a small angle of inclina- 

 tion to a horizontal path, is many times greater than would result from the 

 formula implicitly given by Newton. Thus for an angle of 5° this theoretical 

 vertical pressure would be sin 2 5°cos 5° = 0.0076 of the pressure on a normal 

 plane moving with the same velocity, while according to these experiments it is 

 in reality 0.15 of that pressure, or twenty times as great as the theoretical amount. 



