14© • Sodetf far analyzing MlntraU, i^c- . 



charaSer of minerals often aflifl: in leading to a knowledge of their compofition. It is alfo 

 requefted that an account be fent of the circumftances connefbed with the fpecimen, as the 

 depth below the furface of the earth, and the nature and thicknefs of the flratum in which 

 it is found, and alfo of the incumbent and fubcumbent ftrata, &c. and, where necefTary, 

 fpccimens of thefe with their provincial technical names. — Addrefs the package (carriage paid) 

 to the Society's Secretary, Mr. W. H. Pepys, junior, No. 24, Poultry, London. 



By Order of the Society. 



W. H. PEPYS, Jan. Secretary. 



A Pra£lical Introduction to Spherics and Nautical Aftronomy, being an attempt to fimplify 

 thofe ufeful Sciences, — containing, among other original Matter,the Difcovery of a ProjeiSlion 

 for clearing the Lunar Diftances in order to find the Longitude at Sea ; with a new 

 Method of calculating that important Problem. By P. Kelly, Mafter of Finfbury-fquare 

 Academy, London. Johnfon and Robirrfbns. Price 6s. p. 210. pi. 17. 



This work is. divided into two parts ; the firft contains the principles of flereographic projec- 

 tion, exemplified by numerous figures drawn upon a large fcale ; thefe are followed by right- 

 angled and oblique fphcric trigonometry, in which all the problems are folved both by projeiSion 

 ;ind calculation : this part concludes with improved foliitidns of certain cafes of fpheric triangles. 



,The fecond part treats of aftromony, particularly thofe parts which are ufeful at fea ; fuch 

 as finding azimuths, amplitudes, time, latitude, and longitude. Thefe problems are folved by 

 the globes, and likewife by proje£lion and calculation. 



The method of finding the longitude by lunar obfervation is here explained at fome length, 

 in a clear and elementary manner. The principles of this problem are explained by ftereogra- 

 phic projeftions, whence rules are deduced for eftimating the true diftance beforehand. 



A new projedion here given for clearing the diflance from parallax and refraction, is 

 performed by drawing four right lines from the plan fcale. The author does not infifl upon 

 this method as affording a perfe£l folution of the problem, but he obferves that it will be 

 found fufiiciently exadl for the general purpofes of navigation, and where perfedt correClnefs is 

 required, it will even be found ufeful as a guide or check to calculation. 



The new method given for working the lunar obfervation, confifts in the regular folutiori of 

 two fpherical triangles, and taking the zenith diflances inftead of the altitudes, by which the 

 operation is performed by fines only : tbis is certainly an advantage, efpecially to learners, who 

 are apt to confound fines with cofines, &c. but however fimple and accurate this method may 

 be, it cannot be conveniently applied except by fuch as are in pofTeflion of Taylor's 

 Logarithms, or fome other tables which readily afford portions of the quadrant to feconds. 



The figures throughout this work arc on a large fcale, ^nd appear to be very correft. 



