Physics 549 



Nutation." In this General Barnard attacked mathematically 

 one of the most important problems of recent years, and his 

 conclusion was, essentially in his own words, as follows : 

 " Regarding the crust as rigid, I incline to the opinion of M. 

 Delaunay, that the consideration of the phenomena of preces- 

 sion and nutation can furnish no datum for estimating the 

 greater or less thickness of the solid crust of the earth." The 

 second monograph referred to above, published among the 

 "Smithsonian Contributions" six years later, is, in fact, only 

 supplementary to the third part of the first. In the mean 

 time, much had been said upon the subject, especially by Sir 

 William Thomson, and General Barnard very carefully and 

 conscientiously reviews his own work, and in his conclusion 

 says that " the correction of grave errors of conclusion in 

 papers of mine published under the sanction of the Smith- 

 sonian Institution, and ostensibly deserving the ascription of 

 * Contributions to Knowledge,' is a peremptory motive for 

 this memoir." But he further desires to show that in these 

 papers are to be found essential elements of the correct solu- 

 tion of the " full problem of precession and nutation, and 

 what is now necessarily included in it, the tides, for a contin- 

 uous revolving, liquid spheroid, whether heterogeneous or 

 homogeneous." 



The question is again taken up in a reprint from the 

 Philosophical Magazine, of an article by Henry Hennessy, 

 who had, in 1878, published a paper on the same subject in 

 the same journal. His attitude in the controversy may be 

 suspected from the statement that " geologists are the ultimate 

 judges of the matter, and not mathematicians." In his final 

 sentence he says that the earth cannot consist of an entirely 

 solid mass composed of equi-elliptic strata, and that it is, 

 therefore, partly composed of a solid shell . . . with an in- 

 terior mass of viscid lic^uid, such as is seen flowing from the 



