PAPER BY PROF. HAGEN. 

 According to this, the values of A- are: 



Table IV. 



23 



By connecting among themselves the two first, as also the two last 

 series of observations, the law according to which the value of k de- 

 pends on the size of the disk may be approximately recognized, but 

 the relation between the two forms of disks does not appear clearly. In 

 order to discover this I tried allowing circular and square disks to run 

 one immediately after the other, the radius of the first being 0.5 greater 

 than the side of the latter. From this, however, it could only be in- 

 ferred that for equal areas the resistance of the square disk is the 

 greater. 



In order to recognize the influence of the shape, I tried also disks 

 which formed equilateral triangles of 7.6 inches on each side, which 

 were fastened in such a way that one of the sides stood vertically at 

 the end of an arm. The area of each disk measured 25 square inches, 

 agreeing, therefore, to within a very small quantity, which subsequent 

 accurate measures showed, with that of the square disk of 5 iuches on 

 a side. As I observed these two pair of disks one immediately after 

 the other under the same load, it appeared that the square disk re- 

 volved somewhat more rapidly. This result, however, was not decisive, 

 in that the distances of the centers of pressure from the axis of rota- 

 tion, or R, did not remain the same. In this respect it may be men- 

 tioned that when the side of the equilateral triangle =& and its altitude 

 =h=b cos 30° and the distance of the center of the surface from the 

 axis of rotation is A, we then find 



R=V(A* + ^ti>) 



A complete series of observations, together with the preliminary 

 and the concluding determinations of the value of p and s, gave the 

 following: 



After the computation of 72=98.204, as also after the reduction oiF 



and r there is found 



Ji=2.5026. 



