132 Mr. Airy's Correction in his paper on Diffraction. 



The pressure ;^, corresponding to ^ = — is that part which 



is common to every point of the surface of the sphere. Hence 

 the pressure which tends to put the sphere in motion, or 

 2'-P/> is 



b in r cos 6 cos b t. 

 This, by the remark made at the commencement of the rea- 

 soning, is the same pressui'e as when the fluid is stationary 

 and the sphere in motion, and is identical, if very small quan- 

 tities be neglected, with the value obtained by a very different 

 process at the bottom of p. 465 of my communication to the 

 December Number. The consideration of the lateral press- 

 ure does not therefore lead to any variation from the former 

 result. 



The force of this explanation will, perhaps, be more clearly 

 seen by the following remark. Supposing the sphere at rest 

 and the fluid in motion, the velocity at the position corre- 

 sponding to ^ = is nothing. But as the lines drawn at 

 any instant in the direction of the motion of the particles 

 through which they pass nowhere intersect the surface of the 

 sphere, it follows that the particles about that position are 

 successively carried along the surface till at a position corre- 

 sponding to ^ = — they acquire the velocity of the stream. 



The lateral pressure is concerned in effecting this transfer of 

 the individual particles, while the law of its variation at any 

 instant along the surface of the sphere is that which is given 

 by the preceding investigation. When the fluid is at rest and 

 the sphere in motion the velocity impressed on any particle 

 at the position ^ = is reduced to nothing when it arrives 



at the position ^ = -5^ by the same pressure. 



I am, Gentlemen, yours, &c. 

 Cambridge Observatory, Jan. 18, 1841. J. Challis. 



XXVI. Correction in a paper published in the Philosophical 

 Magazine/or January. By G. B. Airy, Esq., 31. A., F. R. S., 



Astronomer Royal. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



I AM indebted to the kindness of Mr. Tovey for pointing 

 out an omission in my investigation published in your 

 Number for January; arising, as will frequently happen, from 

 the difficulty of attending to every small step when hastily 



