£Q ON THE CONSTRUCTION OF THE HEAVENS, 



Jheir form is generally round ; and the compreffion of the ftars 

 ilievvs a gradual, and pretty fudden accumulation towards the 

 centre, where, aided by the depth of the clutter, which we can 

 have no doubt is of a globular form, the condenfation is fuch, 

 that the ftars are fufficiently comprefled to produce a mottled 

 luftre, nearly amounting to the femblance of a nucleus. A 

 centre of attraction is fo ft rongly indicated, by all the circum- 

 ftances of the appearance of the clufter, that we cannot doubt 

 a (ingle moment of its exiftence, either in a ftate of real foli- 

 dity, or in that of an empty centre, podelled of an hypotheti- 

 cal force, arifing from the joint exertion of the numerous ftars 

 that enter into the compolition of the duller. 



The number of obfervations I have to give relating to this 

 article, in which my telefcopes, efpecially thofe of highfpace- 

 penetrating power, have been of the greateft fervice, of courfe 

 can find no room in this enumeration. 



VII. Of Nebulae. 



7. Nebula. Thefe curious objects, which, on account of their great dif 7 



tance, can only be feen by inltruments of great fpace-pene- 

 trating power, are perhaps all to be refolved into the three 

 laft mentioned fpecies. Cluttering collections. of' ftars, for in- 

 stance, may eafily be fuppoi'ed fur.iciendy removed to prefent 

 us with the appearance of a nebula of any fhape, which, like 

 the real object of which it is the miniature, wiil feem to*be gra- 

 dually brighter in the middle. Groups of ilars alfo may, by 

 diftance, alTume the femblance of nebulous patches ; and real 

 clutters of ftars, for the fame reafon, when their coinpofition 

 is beyond the reach of our molt powerful inftruments to refolve 

 them, will appear like round nebulcc that are gradually much 

 brighter in the middle. On this occafion I mult remark, that 

 with inltruments of high fpace-penetrating powers, fuch as my 

 40-feet telefcope, nebula? are the objects that may be per- 

 ceived at the greateft diftance. Cluttering collections of ftars, 

 much lefs than thofe we have mentioned before, may eafily 

 contain 50,000 of them ; and, as that number has been chofen 

 for an inftance of calculating the diftance at which one of the 

 moft remote objects might be ftill vifible*, 1 fliall take notice 

 of an evident confequence attending the refult of the compu- 



* See Phil. Tranf. for 1800, page 83. N. B In the fame page, 

 line 22, for 5000 read 50,000. 



3 tation ; 



