57 



observation of the angular distance of the two objects to that of the 

 coincidence of their images in one field. In his instrument the te- 

 lescope is fixed at such a distance from the centre that the rays of 

 light may arrive at the centre-glass both from the right and the left : 

 double distances and cross observations are thus easily obtained, 

 which essentially correct any imperfection in the construction of the 

 instrument. 



The peculiar advantages of these improvements being chiefly to 

 afford the means of multiplying the observations of the distance re- 

 quired, Mr. Mendoza has directed his attention to some further im- 

 provements, which he thought might be deduced from the same 

 principle. He accordingly favours us with an account of his new 

 reflecting circle, of which a distinct idea can only be obtained by an 

 inspection of the three accurate delineations which accompany his 

 paper. 



One of the additions is a compound handle, which facilitates the 

 holding the instrument with the same ease in every direction. But 

 the chief improvement appears to be a divided circle, moving round 

 the centre, within, and close to the graduated limb, and capable of 

 being alternately attached to each of the indexes. This the author 

 calls the Flying Nonius ; and shows how in every direction the two 

 divisions may be made to exhibit the number of degrees on the limb, 

 and of the minutes and seconds on the flying nonius. The manner 

 in particular of making the crossed observations, by connecting the 

 limb and the nonius alternately with the centre and horizon indexes, 

 is here fully explained. And lastly, a small graduated semicircle is 

 added to the horizon index, the use of which is to prepare the in- 

 strument previous to an observation, so as to facilitate the operation 

 of bringing the images to coincide in the field of the telescope. 



Observations and Experiments upon Dr. James's Powder ; with a 

 Method of preparing, in the humid Way, a similar Substance. By 

 Richard Chenevix, Esq. F.R.S. M.R.I.A. Read June 4, 1801. 

 [Phil. Trans. 1801, p. 375.] 



From the experiments of Dr. Pearson on the nature of Dr. James's 

 powder, published in the 81st volume of the Philosophical Transac- 

 tions, our author infers that the mode in which it is prepared is far 

 from being the best that the present improved state of chemical 

 knowledge might afford ; the use of fire in delicate processes, whether 

 analytical or synthetical, being in general thought inferior to those 

 performed in the humid way. 



This powder, we are told, is prepared by mixing equal quantities 

 of bone shavings (or phosphate of lime) and crude antimony, and 

 calcining them together in an intense heat. 



Here it is observed, that the portion of oxide of antimony, which 

 is not volatilized in the process, becomes in a great measure insoluble 

 in all acids. The humid process which Mr. Chenevix recommends 

 as preferable to the above, consists in dissolving together or separately, 



