140 Zoological Society. 



remain unchanged, he calculates the change in the whole horizontal 

 refraction which would be produced by two arbitrary and very dis- 

 similar suppositions as to the distribution of temperature in the in- 

 termediate portion of 1075 feet, and draws the conclusion, that thi* 

 variation of refraction arising from any conceivable derangement in 

 this middle portion will be inconsiderable, compared with that which 

 may be produced by changes in the lowest portion. 



In considering the problem of the dip of the horizon, Mr. A. first 

 obtains an expression for it where there is no terrestrial refraction, 

 and deduces this simple approximate formula, that the dip in se- 

 conds = 63"*82 X V~hi where h is the altitude in feet above the 

 level of the sea. 



But when account is to be taken of the terrestrial refraction, 

 Mr. A. finds, that if the included arc of the earth's surface be to 

 the terrestrial refraction as n: 1 (and this is to be determined from 

 a table of terrestrial refraction according to the then existing state 

 of the atmosphere), the preceding expression is thus modified. The 



y~^2, 

 ~~?T X 



ZOOLOGICAL SOCIETY. 



Dec. 28, 1830. W. Yarrell, Esq. in the Chair. 

 The form of a circular letter, to be addressed to the heads of 

 Menageries and Museums in foreign countries, was submitted to 

 the Committee, and approved of. 



A letter was read, addressed to the Secretary of the Society by 

 J. V. Thompson, Esq., dated " Cork, Dec. 16, 1S30." In it Mr. 

 Thompson urges, in support of the universality of a metamorphosis 

 among the Crustacea, that he has ascertained the newly hatched 

 animal to be a Zoea in eight genera of the Brachyura, viz. Cancer, 

 Carcinus, Porlunus, Eriphia, Gecarcinus, Thelphusa?, Pinnotheres, 

 and Inachus ; and in seven Macrourous genera, viz. Pagurus, Por- 

 crllanci) Galathea, Crangon, Palamon, Homarus, and Astacus. 

 " These embrace all our most familiar native genera of the Deca- 

 poda." The Lobster, or Astacus marinus, Mr. Thompson states, 

 " does actually undergo a metamorphosis, but less in degree than in 

 any other of the above-enumerated genera, and consisting in a 

 change from a cheliferous Schizopode to a Decapode ; in its first stage 

 being what I would call a modified Zoea with a frontal spine, spatu- 

 late tail, and wanting the subabdominal fins ; in short, such an ani- 

 mal as would never be considered what it really is, was it not 

 obtained by hatching the spawn of the Lobster." In the other 

 indigenous species of Astacus, Ast.jluviatilis, the River Crawjish, it 

 would appear from the excellent treatise of M. Rathke on the 

 developement of its eggs, that the young are hatched in a form 

 according with that of the fully grown animal. Mr. Thompson, 

 however, suspects that some source of error may exist in these 

 observations. " If it should be found otherwise, it can only be 



regarded 



