438 PHILOSOPHICAL TRANSACTIONS. [anNO 1747-8. 



of the greatest lustre, which it may be presumed lie nearest to us, and may 

 therefore be subject to more sensible changes ; either from their own motion, or 

 from that of our system. And if at the same time that the brighter stars are 

 compared with each other, we likewise determine the relative positions of some 

 of the smallest that appear near them, whose places can be ascertained with suf- 

 ficient exactness; we may perhaps be able to judge to what cause the change, if 

 any be observable, is owing. The uncertainty that we are at present under, 

 with respect to the degree of accuracy with which former astronomers could ob- 

 serve, makes us unable to determine several things relating to the subject that I 

 am now speaking of: but the improvements which have of late years been made 

 in the methods of taking the places of the heavenly bodies, are so great, that a few 

 years may hereafter be sufficient to settle some points which cannot now be 

 settled by comparing even the earliest observations with those of the present age. 



It were to be wished therefore, that such persons as are provided with proper 

 instruments, would attempt to determine, with great care, the present relative 

 positions of several of the principal stars, in various parts of the heavens ; espe- 

 cially of those that are least affected by refraction : that cause having many time* 

 so uncertain an influence on the places of objects that are very remote from the 

 zenith, that wherever it is concerned, the conclusions, deduced from observa- 

 tions that are much affected by it, will always remain dovibtful, and too precari- 

 ous, in many cases, to be relied on. 



The advantages arising from different persons attempting to settle the same 

 points of astronomy near the same time, are so much the greater, as a concur- 

 rence in the result would remove all suspicion of incorrectness in the instruments 

 made use of. For which reason, I esteem the curious apparatus at Shirburn 

 castle, and the observations there taken, as a most valuable criterion, by which I 

 may judge of the accuracy of those that are made at the Royal Obsen'atory ; and 

 as a lover of science I cannot but wish, that our nation abounded with more fre- 

 quent examples, of persons of like rank and ability with lord Macclesfield, equally 

 desirous of promoting this, as well as every other branch of natural knowledge, 

 that tends to the honour and benefit of our country. 



Of Certain Shell-Fish,* Lodged in a large Stone brought from Mahon Harbour. 

 By James Parsons, M. D., F. R. S. N° 485, p. 44. 



Dr. Lister and Rumphius also have figures of this Mediterranean fish, which 

 they call pholas : but since this term barely denotes the place of its residence, 

 let us endeavour to give it a proper name, which may be done by considering its 

 similarity to some genus already known. 



* This shell-fish is the Photos Dactylua of Linnaeus, a common inhabitant of the European rocks. 



