432 PROCEEDINGS OF THE AMERICAN ACADEMY 



Suppose the curve constructed with coordinates equal to x and y of 

 the preceding table, and that a solid of revolution is generated by re- 

 volving it around the axis of T: evidently, the volume of this solid 

 will represent the total amount of light received by the observer from 

 the whole of the sun's disk, and the volume of the circumscribing 

 cylinder will equal that which would be received if the disk throughout 

 had the same brightness as at the centre. The ratio of these two 

 quantities is, however, obtainable by Simpson's formula, and gives the* 

 result 82.6, or the light is about five-sixths of what it would be if the 

 disk had the same brightness at the edges as at the centre. Now, as 

 shown above, the light at the centre is reduced by the atmosphere to 

 26.1 per cent. Hence the total reduction of the whole surface is 

 .261 X .826 = .2 1 6. And, since the light is reduced in every direc- 

 tion by the same amount, we may say that the sun would give out 4.64 

 times as much light if its atmosphere were removed. 



The results of this paper may therefore be summed up as follows. 

 The light of the various parts of the sun's disk is measured by the 

 modification of the Bunsen photometer here employed, and given in the 

 accompanying table, with a probable error not exceeding one per cent 

 except close to the edge. The light at the edge is about .4 of that at the 

 centre. The variations in brightness are nearly those which would be 

 produced by a homogeneous atmosphere of height equal to the sun's 

 radius, and opacity such that only 26 per cent of the light is trans- 

 mitted. There appears to be a slightly different distribution of the 

 light along the polar, from that along the equatorial, diameter. If the 

 atmosphere were removed, the brightness of the sun's disk would be 

 uniform, and 3.83 times that of the centre of the disk at present. 

 Moreover, the total amount of light would be increased 4.64 times. 



VI. — TESTS OF A MxVGNETO-ELECTRIC MACHINE. 

 By E. C. Pickering and D. V. Strange. 



The rapidly increasing use of magneto-electric machines as a source 

 of electricity renders accurate tests of the comparative advantages of 

 the vaiious forms and exact measurements of the currents generated 

 under varying conditions very desirable. The machine employed in 

 the following experiments was made by Mr. M. G. Farmer, and con- 

 sists of ii large electro-magnet wound with four coils soldered together 



