ANNIVERSARY MEETING.— WOLLASTON MEDAL. XXV 



parallelism, and at the end of the year the inclination of the axis is 

 not exactly as at the beginning. This produces a motion of the 

 equinoxes of about fifty seconds annually in a direction contrary to 

 the order of the signs of the zodiac. Previously to your researches, 

 it had been proved that this motion might be exactly accounted 

 for, by supposing the earth to have been primitively fluid, and to 

 have at length become perfectly solid to its centre. But you have 

 inquired whether the precession of the equinoxes would be such as it 

 is observed to be, if the earth consisted of a solid shell covering an 

 internal fluid nucleus. You point out that the mechanical action of 

 the attractions of the sun and moon must, in that case, be entirely 

 diff'erent from the case of a solid globe ; and by your calculations we 

 are taught, that if the crust were as thin as is supposed by many 

 geologists, when they reason on the causes of volcanos, the preces- 

 sion would differ from its observed amount by a quantity which could 

 not be mistaken. Hence you draw the important inference that the 

 minimum thickness of the crust of the globe cannot be less than one- 

 fourth or one-fifth of the earth's radius ; that is to say, from 800 

 to 1000 miles. It is, I believe, generally felt by mathematicians, 

 that a knowledge of the possible influence of great pressure in pro- 

 moting solidification is a great desideratum, before we can carry out 

 these speculations on the internal fluidity of the earth to their full 

 extent ; and we rejoice therefore to hear that you are now engaged 

 in experiments which lead you to hope that you may be able ere 

 long to supply this deficiency. 



Mr. Hopkins said, in reply, — 



I beg, Mr. President, to express my sense of the honour which 

 the Council of the Geological Society has conferred on me by their 

 award of the Medal which you have just presented to me. Few 

 persons, I believe, can have entered on and prosecuted the study of 

 geology impelled more than myself by a spontaneous love of the sci- 

 ence. The investigations and speculations which it presents to us 

 have always been in my estimation of especial interest ; and if I had 

 met with no reward besides the pleasure which has attended the pro- 

 secution of such researches, I should still have thought the time and 

 labour I have devoted to them well bestowed. But, Sir, every one 

 who has been long and earnestly engaged in any scientific inquiry, 

 will be well aware how much gratification and encouragement we 

 may derive from the sympathy of those who are fellow-labourers 

 with us in the same field of investigation. The expression of this 

 sympathy on the part of the Geological Society towards myself, in 



