1821.] Astronomical Society/. 393 



read. Its advantages over others are obtained by applying a 

 transverse axis to the telescope and dedination circle, thereby 

 giving them all the perfections of the transit instrument, without 

 diminishing their repeating properties ; and by the application of 

 two levels alternately becoming finders to the telescope, which 

 render the back semicircle unnecessary, a contrivance which 

 saves much loss of time when repeating zenith distances. The 

 instrument is also constructed to repeat in azimuth, and is fur- 

 nished with a very dehcate level, as a check upon the horizontal 

 circle, and which answers all the purposes of an under telescope. 

 The object glass of the telescope is of two inches diameter, and 

 17 inches focal length : with it the pole star may be distinctly 

 observed in the day time. The angles are read off to every 

 tenth second by verniers and fixed microscopes. The instrument 

 differs from all others previously made ; is furnished with every 

 requisite for ascertaining vertical and horizontal angles, and 

 which, if taken sinj^ly, may be, in the author's opinion, as 

 correctly obtained by this, as by any instrument of the same 

 dimensions not possessing the repeating principle. 



Article XIII. . 



SCIENTIFIC INTELLIGENCE, AND NOTICES OF SUBJECTS 

 CONNECTED WITH SCIENCE. 



I. Purple Po'wder of Cassias. 



Dr. Clarke, of Cambridge, has recently examined the purple pow- 

 der of Cassius ; and from his experiments, he concludes, that the binary 

 compound which he analysed consisted of the oxides of tin and gold, 

 and contained these oxides chemically combined in the exact propor- 

 tion of three parts of tin to one of gold; and that the alloy of the two 

 metals obtained by the fusion of 100 parts of the purple powder 

 would yield 



Metallic tin 75 



Metallic gold 25 



100 

 Because 8-1 Oths of a grain of the alloy yielded 



Metallic tin O'S 



Metallic gold 0-2 



0-8 



Dr. Clarke also infers, that in precipitating the purple powder of 

 Cassius from the muriate of gold, by means of the muriate of tin, the 

 two metals, tin and gold, are thrown down as oxides, which, however, 

 do not chemically combine in a constant relative proportion to each 

 other ; that the quantity of tin always exceeds that of the gold ; and that 



