208 
MR. HOPKINS’S RESEARCHES IN PHYSICAL GEOLOGY. 
The coefficients in the terms which give the nutation are altered by the hypothesis 
of the earth’s heterogeneity in the same ratio as the coefficient of precession. 
If we take the ellipticity of the earth = the calculated value of Pj is nearly 57", 
while the observed value of P' is only 50"- 1. Consequently the value of the multi- 
plier of ?! in the above expression for Pj — P' ought to equal nearly If, then, the 
value of s, the ellipticity of the inner surface of the solid shell, be so much less than q 
as to satisfy this condition, precession and nutation may be as well accounted for on 
the hypothesis of the earth’s internal fluidity, as on that of its entire solidity. The 
required difference, however, between i and q is such as to indicate that the least 
thickness of the earth’s crust, compatible with the observed amount of precession, is 
considerable. 
The discussion of the evidence which the results of the preceding investigations 
afford on this point, combined with other considerations, must be reserved as the 
subject of a future communication to the Society. 
Cambridge , 
Feb. 13, 1839. 
