VOL. LXXXV.] PHILOSOPHICAL TRANSACTIONS. 50)3 



bable that it is then plunged in cold water, the great hardness of the pieces im- 

 ported, above that of our steel, must be imputed to its containing oxide, and 

 consequently oxygen. The particular uses to which wootz may be applied, may 

 be inferred from the preceding account of its properties and composition : they 

 will also be discovered by an extensive trial of it in the innumerable arts which 

 require iron. 



XVllI. Description of a Forty-feet Reflecting Telescope. By JVm. Hefschel, 



LL.D. F.R.S. p. 347. 



It will be necessary to mention a few circumstances that led the way to the 

 construction of this large instrument, in the execution of which 1 very material 

 requisites were necessary : namely, the support of a very considerable expence, 

 and a competent experience and practice in mechanical and optical operations. 

 When I resided at Bath I had long been acquainted with the theory of optics and 

 mechanics, and wanted only that experience which is so necessary in the practi- 

 cal part of these sciences. This I acquired by degrees at that place, where in my 

 leisure hours, by way of amusement, I made for myself several 2-feet, 5-feet, 

 7-feet, 10-feet, and 20-feet Newtonian telescopes; besides others of the Gre- 

 gorian form, of 8 inches, 12 inches, 18 inches, 2 feet, 3 feet, 5 feet, and 10 

 feet focal length. My way of doing these instruments at that time, when the 

 direct method of giving the figure of any of the conic sections to specula was still 

 unknown to me, was, to have many mirrors of each sort cast, and to finish them 

 all as well as I could ; then to select by trial the best of them, which I preserved ; 

 the rest were put by to be re-polished. In this manner I made not less than 200, 

 7-feet; 150, 10-feet; and about 80, 20-feet mirrors ; not to mention those of 

 the Gregorian form, or of the construction of Dr. Smith's reflecting microscope, 

 of which I also made a great number. 



My mechanical amusements went hand in hand with the optical ones. The 

 number of stands I invented for these telescopes it would not be easy to assign. I 

 contrived and delineated them of different forms, and executed the most pro- 

 mising of the designs. To these labours we owe my 7-feet Newtonian telescope- 

 stand, which was brought to its present convenient construction about 1 7 years 

 ago ; a description and engraving of which I intend to take some future oppor- 

 tunity of presenting to the r. s. In the year 1781, I began also to construct a 

 30-feet aerial reflector ; and after having invented and executed a stand for it, I 

 cast the mirror, which was moulded up so as to come out 36 inches in diameter. 

 The composition of my metal being a little too brittle, it cracked in the cooling. 

 I cast it a 2d time, but here the furnace, which I had built in my house for the 

 purpose, gave way, and the metal ran into the fire. These accidents put a tem- 

 porary stop to my design, and as the discovery of the Georgian planet soon after 



VOL. XVII. 4 G 



