analysis; 247 



dal, it may be requifile, before any intricate refearches are 



attempted (as mentioned by the Dodof at the end of the 



communication), to be well aflured that his telefcopes have 



defined ihe figures of the planets accurately, which at prefent 



admits of a doubt, and which may be cleared up about the 



time of the next oppofition of the Sun and Saturn, in April . 



next. 



The following may prove the neceffity of fuch an enquiry : 



Place a circular or fpherical figure before a concave mirror. Experiment. 



which mirror muft be fo inclined, that when the objedt '^^ kYolhqiien^ 



above the head of the obferver, it may be feen, by refledlion, flcftion ftom a 



in the center of the mirror*: If feen within the focus, thefP^^'T'f'f » 



' IS rendered «>- 



object will be reprefenfed oval in a vertical direftion, and long, 



when beyond the focus, in a horizontal; which figure will be 



more and more oval as the angle is enlarged. 



Your's, 



An observer. 



XVI. 



Experiments on a Mineral Subftance formerly fuppofed to be 

 . 'Zeolite ; with fame Remarks on two Species of Uran-glitnmer, 

 By the Rev. William GuEcoR.f 



Ji- HIS mineral is raifed in a mine called Stenna Gwyn, in Defciiption and 

 the parilh of St. Stephen's, in Branwell, in the county of Jl^j^J^J^froA 

 Cornwall ; the principal produ6tion of which is the compound Cornwall. 

 fulphuret of tin, copper, and iron, 



Defcription. 



Two fpecies of this mineral are found, afluming a marked 

 difference in external charader. 



The firft and moft common one confifts of an affemblage of 

 Aiinute cryftals, which are attached to quartz cryftals, in tufts, 

 which diverge from the point of adherence, as from a centre, 

 Thefe tufts vary, as to the number of cryftals, of which they 



* If the objeft is fmall, it may be enlarged by a concave eye- 

 glafs. 



t Phil. Tranf. 1805, 



are 



