Page:Proceedings of the Royal Society of London Vol 1.djvu/77

Rh 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 рарer.

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 showe 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, hy 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.

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,