526 
MR. HENNESSY’S RESEARCHES IN TERRESTRIAL PHYSICS 
with the fluid matter, but here we eannot consider the transition from solidity to 
fluidity as abrupt. Many physical analogies lead us to believe that, on the contrary, 
this transition must be gradual, or that a stratum of matter not completely fluid 
must exist between the highly fluid matter of the nucleus and the solid matter of the 
shell. The transition from this imperfectly fluid stratum to the perfectly fluid matter 
within it; will be also gradual, but it is possible to conceive that a portion of it in 
immediate contact with the shell may merely possess that kind of fluidity which in 
solids would be considered extreme softness. This matter will cohere to the shell 
with a considerable force, compared to the cohesion of the fluid particles ; and this 
fact, combined with its viscidity, must serve to remove any doubt as to its not being 
subjected to the same hydrostatical laws as the perfect fluid. If now a surface be 
conceived to exist which may be called the effective surface of separation of the per- 
fect from the imperfect fluid, it is evident that its form will depend on the pressures 
which the former tends to exert. 
Let AEBF in the accompanying figure represent the profile of the inner surface 
of the shell, aebf of the similar and concentric surface of the perfect fluid. In 
this case the pressure exerted by the nucleus would be the same for all points of the 
shell, and the thickness of the stratum of imperfect fluid would be everywhere the 
same. If, however, the pressure of the perfect fluid were not constant/ its surface 
would tend to assume a different form, in order to re-establish equilibrium ; it might 
