Royal Society of London.. 3at, 



fcach a centre, fo as to exhibit a mottled luftrc, almoft refem-» 

 bling a nucleus. The eighth fpecies confifts of nebulae, 

 which probably differ from the three laft fpecies only in being 

 much more remote; fome of them, Dr. Ilerfchel calculates, 

 in uft be at fo great a diftance that the rays of light muft have 

 been nearly two millions of years in travelling from them to 

 our fy(tem. The ftcllar nebula*, or (tars with burs, form a 

 diftinct fpccies. A milky nebulofity is next mentioned, 

 which may in fome cafes referable other nebula?, but in 

 others appears to be diflufed, almoft like a fluid : the author 

 is not inclined to coniider it as either rcfembling the zodiacal 

 light of the fun, or as of a phofphorefcent nature. The tenth 

 fpecies is denominated nebulous (tars; thefe are flars fur- 

 rounded with a nebulofity like an atmofphere, of which the 

 real magnitude muft be amazingly great; for the apparent 

 diameter of one of them, defcribed in the catalogue, was 3'. 

 The planetary nebulae are diftinguiftied by their equable 

 brightnefs and circular form, while their light is ftill too 

 faint to be produced by a (ingle luminary of great dimenfions. 

 When they have bright central points, Dr. Herichel consi- 

 ders them as forming a twelfth fpecies, and fuppofes them to 

 be allied to the nebulous ftars, which might approach to their 

 nature, if their luminous almofpheres were very much con- 

 denfed round the nucleus. 



On the 8th of July, the firft part of a paper on the rectifi- 

 cation of the conic feclions was laid before the Society by. 

 the ttev. John Hellins, B.D. F.R.S. It contained nine theo- 

 rems for the rectification of the hyperbola by means of in- 

 finite fcries, one only of which had been before publiilied, 

 each having its particular advantages in particular cafes of 

 the proportions of the axis and of the ordinates, fo that they 

 appear to contain a complete practical folution of this import- 

 ant problem, and they are iltaftrated by a variety of examples. 



On the fame evening were read obfervations on heat, and 

 on the action of bodies which intercept it, by Mr. Prevoft, 

 profeffor of natural philofophy at Geneva. They confifl 

 -chiefly of inferences from Dr. Ilerfchel's important experi- 

 ments on the traulmiflion of heat by different refracting me- 

 diums, efpecially the different kinds of glafs. Mr. Prevoft 

 fets out with the law of the interchange oi heat as afcertained 

 by the experiments of MM. Kraft and Richmann, that while 

 the time flows equably, the differences of the temperature of 

 two contiguous bodies flow proportionally, or are in geome- 

 trical progrcmon. Hence from three obfervations of the 

 actual temperature of a thermometer, at given intervals of 

 time, .we may determine the progreflion of the differences, 

 and confequently the actual heat of the medium. The au- 

 4 thor 



