EECEEATIVE SCIENCE. 



853 



the lead appears infinitely brighter than the 

 surrounding furnace, a result which is due to 

 the intense heat produced by oxidation. Sud- 

 denly, as less oxidizable metal remains, the 

 surface becomes dull, when, with lightning- 

 like rapidity, there is another change. The 

 gradually thinning film of oxide presents a 

 quick succession of prismatic colours, until 

 presently, as a rainbow fades from a sunlit 

 lake, it breaks, and the bright sheet of silver 

 comes into view. Twenty tons of lead are 

 usually cupelled at one operation, and, con- 

 sequently, the cake of silver produced weighs 

 about 6000 ounces. 



Before we bid farewell to lead, let us 

 visit yonder tower upon the hill, and see how 

 shot are made. But, first of all, there is a 

 certain furnace at which we must glance. It 

 is in a building which adjoins the cupelling 

 hou«e, and close beside it lies a large heap of 

 a heavy white powder. There are also some 

 pigs of lead, which are of a darker colour 

 than any which we have yet seen ; when you 

 strike them, they make a sharp ringing 

 sound. These pigs are the lead from the 

 slag, or scoria, which, as we saw just now, is 

 reserved for shot-making, seeing that it is 

 almost worthless for any other purpose. The 

 white powder is arsenious acid — the white 



arsenic of the shops. What is this for ? If 

 you melt lead, and pour it into water through 

 a metal sieve, it will form a number of irre- 

 gularly-shaped masses, not at all globular, or 

 even ellipsoid. But if you try the sa'tae 

 experiment with lead to which a little arsenic 

 has been added, the drops on cooling will be 

 nearly perfectly spherical. The arsenic is 

 added to the melted lead in a very small pro- 

 portion, not more than from seven to ten 

 parts to one thousand, and its oxygen uniting 

 with some of the lead,- forms litharge, which 

 floats at the top, while the liberated arsenic 

 forms an alloy with the remainder of the 

 metal. This alloy of lead and arsenic, or, 

 as it is technically called, " poisoned metal," 

 is next taken to the shot-tower, where 

 it is remelted and poured from the summit 

 through perforated metal plates, or " cards," 

 as they are called. It falls into a vessel 

 of water placed to receive it, and the sub- 

 sequent operations which the drops have to 

 undergo, before they find their way into the 

 shot-belt of the sportsman, are but few and 

 simple. They comprise the assortment of 

 the drops into vaTiou* sizes, the removal 

 of such as are not perfectly spherical, and 

 thfe polishing by means of plumbago. 



Haeey Napiee Deapee, F.C.S. 



A MERIDIAN-LINE. 



To possess some independent means of ob- 

 taining the true time, whereby a person may, 

 in his own house, ascertain the error of his 

 clock or watch to within a second or two, is 

 80 obvious a desideratum, that all readers of 

 Heceeative Science, who do not at present 

 possess the means, will, doubtless, be pleased 

 to have their attention directed to a simple 

 method by which so desirable a result may 

 be obtained. 



The transit-instrument, the sine qua 

 non of observatories, is, doubtless, the best 

 for the purpose. It is, however, rather 

 an expensive instrument, and for accurate 

 purposes requires frequent and careful ad- 

 justment. 



The sun-dial is too rough a method. By 

 it the time can hardly be obtained nearer 

 than to within a minute of the truth. There 

 are other methods, many of which are inte* 



