PllOCEEDlNGS OF THE ROYAL SOCIET?. 



681 



of some bei4ig direct, and of others retrograde. The pro- 

 per motion of our sun does not appear to be of this kind, 

 but to be rather the effect of some perturbations in the 

 neighbouring systeins. Tlie same theory is next applied to 

 triple, quadruple, and multiple systems of stars, and parti- 

 cular hypothetical cases arc explained by diagrams. Some 

 such cases. Dr. Herschel is fully persuaded, have a real 

 existence in nature. The fourth species consists of cluster- 

 ing stars, and of the milky way : the stars thus disposed 

 constitute masses, which appear brighter in the middle, ^ 

 and fainter towards the extremities, being perhaps collected 

 in a spherical form. Groups of stars the author distin- 

 guishes from these by a want of apparent condensation 

 about a centre of attraction : and clusters of stars, by a 

 much more complete compression near such a centre, so as 

 to exhibit a mottled lustre, almost resembling a nucleus. 

 The eighth species consists of nebulae, which probably 

 differ from the three last -species only in being much more 

 remote ; some of them. Dr. Herschel calculates, must be 

 at so great a distance, that the rays of light must have been 

 nearly two millions of years in travsUing from them to our 

 system. The stellar nebulae, or stars with burs, form a 

 distinct species. A milky nebulosity is ne.tt mentioned, 

 which may in some cases resemble other nebulae, but in 

 others appears to be diffused, almost like a fluid : the 

 author is not inclined to consider it as either resembling the 

 zodiacal light of the sun, or of a phosphorescent nature. 

 The tenth species is denominated nebulous stars ; these are 

 stars surrounded with a nebulosity like an atmosphere, of 

 which the magnitude must be amazingly great ; for the 

 apparent diameter of one of them, described in the catalo- 

 gue, was 3'. The planetary nebulae are distinguished by 

 their equable brightness, and circular form, while their light 

 is still too faint to be produced by a single luminary of great 

 dimensions. When they have bright central points. Dr. 

 Herschel considers them as forming a twelfth species, and 

 supposes them to be allied to the nebulous stars, which 

 might approach to their nature, if their luminous atmo- 

 spheres were very much condensed round the nucleus. 



On the 8th of July, the first part of a paper on the recti- 

 fication of the conic sections was laid before the society by 

 the Rev. John Hellins, B.D. F. R. S. It contained nine 

 theorems for the rectification of the hyperbola, by means 

 of infinite series, one only of which had been before pub- 

 lished, each having its particular advantages, in particular 

 cases of the proportions of the axes and of the ordinatcs, so 

 that they appear to contain a complete practical solution of 

 this important problem, and they are illustrated by a va- 

 riety of examples. The author obseives that Dr. Waring's 

 theorems, for computing the length of the curve, from ordi- 



VOi« I. 



nates referred to the asymptote, are in their present form of 

 little use, but might easily be corrected in a manner similar 

 to that which he has pursued. He defers, to a future oppor- 

 tunity, the publication of similar investigations relative to 

 the ellipsis. 



Observations on Heat, and on the action of bodies which 

 intercept it. By Mr. Prevost, Professor ofN<nuial Philo- 

 sophy at Geneva. 



This paper was read on the same evening. It consists 

 thiefiy of inferences from Dr. Herschel's important experi- 

 ments on the transmission of heat by different refracting me- 

 diums, especially the difl'crent kinds of glass. Mr. Prevost 

 sets out with the law of the interchange of heat, as ascer- 

 tained by the experiments of IVIM. Kraft and Richmann, that 

 while the time flows equably, the differences of the tem- 

 peratuft of two contiguous bodies flow proportionally, or 

 are in geometiical progression. Hence, from three observa- 

 tions of the actual temperature of a thermometer, at given 

 intervals of time, we may determine the progression of the 

 differences, and consequently the actual heat of the medium . 

 The author applies this method to Dr. Herschel's experi- 

 ments on the heat of a solar ray tiaiismitted through dif- 

 ferent mediums, and the conclusions are very different 

 from what we should at first sight infer : for instance, 

 in Dr. Herschel's a4th experiment, the blue glass inter- 

 cepted one tenth only of the rays of heat, and not one fourth, 

 as the thermometer seemed to indicate. But the imme- 

 diate interception must have been somewhat greater than 

 one tenth; for a certain portion of heat, actually communi- 

 cated to the glass, must hare radiated afresh towards the 

 thermometer, and contributed W produce the temperature 

 observed ; and accordingly as this circumstance took place 

 in a greater or less degree, the thermometer must have 

 been variously and irregularly affected. Of such an irregu- 

 larity almost every one of the experiments shows evident 

 marks, and the app^atus is not minutely enough described 

 to furnish data for calculating its magnitude. From these 

 principles an experiment of Mr. Pictet, on the interception 

 of heat, is reconciled with Dr. Herschel's experiments. 



In the second part of this paper, Mr. Prevost treats of the 

 reflection of heat and of cold. He observes tliat Bacon sug- 

 gested the inquiry respecting the concentration of invisible 

 heat by glasses. Lambert attributed the effect of the re- 

 flection from a common fire to its invisible heat. Mr. de 

 Saussure suggested to Mr. Pictet to confirm Lambert's sus- 

 picion by experiment, and the success is well known. His 

 experiment on the reflection of cold Mr. Prevost has al- 

 ready employed in support of the opinion that the cquili- 

 brium of heat is not a quiescent equilibrium, or an equili- 

 brium of tension, but an equilibrium of motion, where the 



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