194 Archdeacon Pratt on the Thickness of 



graphy (non-commissioned officers, Royal Artillery) were found 

 extremely valuable in conducting the fixing and subsequent 

 operations, taking from my hands the plate immediately after the 

 development of the picture. 



In viewing the photographs thus produced, it may be stated 

 that several of them exhibit very clearly the position of the solar 

 spots, and that in many cases the lunar mountains are to be seen 

 sharply defined against the bright face of the sun. A passing 

 cloud rendered the " first contact " so imperfectly visible that no 

 opportunity for opening the camera was presented ; but the plate 

 representingthe moment of maximum obscuration, and the phases 

 both immediately preceding and following, are fortunately amongst 

 the most successful of our photographic results ; and further, on 

 comparing the present series with the disc of the sun taken at 

 the period of the former eclipse, March 15th, 1858, in precisely 

 the same apparatus, the diminished size of the image consequent 

 on the eccentricity of the earth's orbit is very strikingly exhibited. 



As a concluding remark, I beg to call attention to the unusual 

 magnitude of a group of spots visible upon the sun's disc on the 

 9th instant. They were readily seen without the assistance of 

 the telescope, as a dark patch situated rather low in the southern 

 hemisphere. I have succeeded in securing three concordant 

 photographic proofs representing their appearance at noon and a 

 little later, which, besides exhibiting the principal group as an 

 aggregate of dark spots disposed generally in a horizontal line, 

 indicate likewise the position of several smaller ones irregularly 

 distributed over the face of the sun, and only visible with optical 

 aid. It cannot be said that any distinct indications of faculse 

 are afforded by these or the other photographs taken on the 

 occasion of the eclipse. 



Royal Arsenal, Woolwich, 

 August 21st, 1860. 



XXV. On the Thickness of the Crust of the Earth. 

 By the Venerable John Henry Pratt, Archdeacon of Calcutta. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



I ADMIT that I did use the principle pointed out by Professor 

 Jellett in the note to his communication in your May 

 Number, and in so doing was wrong ; and therefore the part of 

 my argument against Professor Haughton's investigation in the 

 Irish Transactions, which depends upon that step, falls to the 

 ground. 



But this does not affect the objection which I brought forward 

 in two of my communications (May 1859, p. 329, and October 



