174 PHILOSOPHICAL TRANSACTIONS. [aNNO 1674. 



II. The Second Book of the Art of Metals, wherein is taught the common 

 way of Refining Silver by Quicksilver, with some New Rules added for the 

 better performance of the same : written in Spanish by Alonso Barba, and 

 Englished by the Right Honourable Edward Earl of Sandwich, London^ 1674, 

 in 8vo. 



The first book of this subject was noticed in the last foregoing Number. 

 This second consists of 24 chapters, most of them of a practical nature; de-_ 

 scribing the methods employed by the workmen in extracting and refining the 

 metal. 



III. Animadversions on the First Part of the Machina Coslestis, of the de- 

 servedly famous astronomer Johannes Hevelius, &c. together with an Explica- 

 tion of some Instruments, made by Robert Hook, P. of Geom. in Gresh. Col. 

 and F. R. S. London, 1674. 



This piece consists, as the title intimates, of two main parts. The one repre-- 

 sents the author's thoughts of the astronomical organography of the excellent 

 Hevelius, both examining his, and Tycho Brahe's instruments, and undertaking 

 to evince, that if they had made use of telescopical sights, their observations 

 might have been much more exact than they are; forasmuch as, in the author's 

 opinion, an instrument of 3 feet radius with telescopes, will do more than one 

 of 60 feet radius with common sights. The other describes an instrument for 

 taking all sorts of angles and distances in the heavens, which, if increased in 

 bulk, is capable, Mr. Hook says, of as great accuracy, as the atmosphere will 

 ever permit celestial observations to be made. Its perfection he places in these 

 seven particulars : 1 . In the sights ; which are such, as may be made to discover 

 the minutest part discoverable in an object, not at all straining the eye, and fit 

 for all eyes. 2. In the divisions; which are such, as will distinguish the angle 

 as minutely as the sights will distinguish the parts or objects, and that even to 

 seconds. 3. In such a contrivance of the sights, that with one glance of the 

 eye, both the objects, though a semi-circle distant, are at once distinguished 

 and seen together. 4. In the method of setting it exactly perpendicular to a 

 second if need be. 5. In its fixation and motion; it being so fixed and moved, 

 that if once set to the objects, it continues to move along with them, as long as 

 it is necessary to continue, or be very certain of any observation. 6. In its 

 being to be made and adjusted without difficulty, and not to be put out of order 

 without design ; as also in its great easiness of being rectified and again adjusted, 

 7. In its being not very chargeable. 



All these perfections the author explains, and endeavours to make good, by 

 describing and delineating this instrument and all the parts thereof, and endea- 

 vouring to obviate such exceptions as he foresaw might be made against it. 



