1873.] On a supposed Alteration in the Aberration of Light. 121 



variable x ; but the three angular velocities are equivalent to a single an- 

 gular velocity, and then, as we see in article 4, we shall have involving 

 also y and z. 



6. The problem which Dahlander discusses is in reality much simpler 

 than his enunciation implies ; it amounts to this : investigate the condi- 

 tions under which a fluid will be in equilibrium in the form of an ellip- 

 soid, when, besides the attraction of the fluid, there are forces parallel to 

 the principal axes, which may be denoted by fx, gy, hz respectively. 

 Traces of such a problem appear in other places — as, for example, in 

 Lagrange's ' Mecanique Analytique,' premiere partie, Sect. VII. This is, 

 however, different from the problem of rotating fluid, which it was pro- 

 posed to discuss. 



7. There is nothing to call for remark in the mathematical work of the 

 memoir, except that a wrong value is assigned to the definite integral 



f (l— u )u du ^ correct value is 



r- (l + W)* 



3 + ^ log{x+A/(1 + x2)} _3V(l + X 2 ) 



2K 5 2k 4 

 October 24, 1872- 



III. Additional Note to the Paper " On a supposed Alteration in 

 the Amount of Astronomical Aberration of Light produced 

 by the Passage of the Light through a considerable thickness 

 of Refracting Medium/' By the President. Received No- 

 vember 2, 1872. 



Some months since I communicated to the Royal Society the result of 

 observations on y Draconis made with the water-telescope of the Royal 

 Observatory (constructed expressly for testing the equality of the coeffi- 

 cient of sidereal aberration, whether the tube of a telescope be filled with 

 air, as usual, or with water) in the spring and autumn of 1871. Similar 

 observations have been made in the spring and autumn of 1872, and I 

 now place before the Society the collected results. It will be remem- 

 bered, from the explanation in the former paper, that the uniformity of 

 results for the latitude of station necessarily proves the correctness of 

 the coefficient of aberration employed in the Nautical Almanac. 



Apparent Latitude of Station. 



1871. Spring 51 28 34-4 



Autumn 51 28 33-6 



1872. Spring 51 28 33-6 



Autumn 51 28 33-8 



I now propose, when the risk of frost shall have passed away, to 

 reverify the scale of the micrometer, and then to dismount the instrument. 



VOL. XXI. L 



