280 Archdeacon Pratt on the Thickness of 



fluence of the Gulf-stream as a heating-agent has been so much 

 underestimated. 



The same remarks apply to the gravitation-theory of the cause 

 of currents. Viewed simply as a theory it looks very reasonable. 

 There is no one acquainted with physics but will admit that the 

 tendency of the difference of temperature between the equator 

 and the poles is to cause a surface- current from the equator 

 towards the poles, and an under-current from the poles to the 

 equator. But before we can prove that this tendency does actu- 

 ally produce such currents, another question must be settled, 

 viz. is this force which tends to produce the motion sufficiently 

 strong to overcome the resistance of the water to go into motion? 

 Dr. Carpenter has never attempted to prove that it is ; he has 

 simply taken it for granted. But when we apply the method 

 to which I refer, and determine the absolute amount of the force 

 resulting from the difference of specific gravity, we find it to be 

 not that powerful thing which the advocates of the gravitation- 

 theory suppose it to be, but a force so infinitesimal as not to be 

 worthy of being taken into account when considering the causes 

 which produce currents. 



[To be continued.] 



XXX. The Solid Crust of the Earth cannot be thin, 

 By Archdeacon Pratt, M.A., F.R.S. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



MDELAUNAY considers that the Solid Crust of the Earth 

 • may be thin — and that, if it be so, the fluid nucleus 

 moves exactly as the crust does in precession and nutation, in 

 consequence of the viscosity of the fluid and the extreme slow- 

 ness of the motion — and that, in consequence, the late Mr. W. 

 Hopkins's argument from precession that the crust is thick, at 

 least 800 or 1000 miles, does not stand examination. I have 

 already forwarded two letters to you on this subject ; and now 

 ask you to publish a third, to show why the slowness of the mo- 

 tion has nothing to do with the question. 



The particles of the earth's mass have much more to undergo 

 than performing this slow motion. They have during the 

 twenty-four hours of the earth's rotation to undergo a varying 

 strain which no fluid, even viscous, could sustain. The force 

 producing the slow motions of precession and nutation is the 

 result of the near balance of a multitude of forces acting through- 

 out the mass, almost equal and opposite to each other. Suppose 

 of two fluid molecules in contact one only is acted on by a minute 



