65 



entering into intimate union with the oxide of platinum, produces 

 fulminating platinum. The theories invented by Bergmann and Ber- 

 thollet, to explain the detonation of fulminating gold, are satisfac- 

 torily applicable to the phenomena presented by the compound now 

 described, the explosive powers of which may be referred to the 

 sudden extrication of nitrogen, ammonia, and aqueous vapour. 



On the Parallax of the fixed Stars. By John Pond, Esq., Astrono- 

 mer Royal, F.R.S. Read February 20, 1817. [Phil. Trans. 1817, 

 p. 158.] 



Dr. Brinkley, of the Observatory of Dublin, having noticed for 

 several years past a periodical deviation of several fixed stars from 

 their mean places, strongly indicating the existence in them of an- 

 nual parallax, the author was induced to institute a series of obser- 

 vations upon the subject, the results of which are submitted to the 

 Royal Society in the present communication. Being unable to de- 

 vote the mural circle, erected at the Royal Observatory in 1812, 

 entirely to this investigation, the Astronomer Royal employed two 

 ten-feet telescopes, fixed to stone piers, and directed to the parti- 

 cular stars whose parallax was suspected, and furnished with micro- 

 meters for the purpose of comparing them with other stars passing 

 through the same field. The question of parallax is, theoretically 

 speaking, rather curious than important; but with regard to the 

 state of practical astronomy the case is very different, and, as far as 

 relates to the natural history of the sidereal system, it is a subject of 

 interest to ascertain whether the distances of the nearest fixed stars 

 can be numerically expressed from satisfactory data, or whether it 

 be so immeasurably great as to exceed all human powers either to 

 conceive or determine. 



The principal stars observed by Dr. Brinkley were, a Lyrae, 

 a Aquilae, a Cygni. 



The mean of forty observations of a Lyrae, made by the Astro- 

 nomer Royal between June 22 and August 21, gave for the north 

 polar distance of that star 5 1 23' 0"'278. The mean of twenty ob- 

 servations nearer the period of opposition gave 51 23' 0" - 468. The 

 mean of thirty winter observations is 51 23' 0"'872. The discord- 

 ance, therefore, between the winter and summer observations does 

 not exceed 0"'6, which is only one third the discordance observed 

 by Dr. Brinkley. 



With a Cygni the total discordance in favour of parallax was 

 0"'556; also only one third that observed by Dr. Brinkley; and 

 with Aquilae it is less than 0"'5, equal only to one fourth of the 

 discordance observed by Dr. Brinkley. 



From these and other observations detailed in the paper, the 

 Astronomer Royal observes, that in the three stars supposed by Dr. 

 Brinkley to have the greatest parallax, the discordance between the 

 summer and winter observations is not less than 0"'5, and scarcely 

 exceeds 0"*75 : and that although these quantities are much less 



