Mechanical Science* 471 



5. Tinning of Cast Iron Weights. — The weights are first to be 

 cleaned in diluted sulphuric acid of specific gravity 1.14 or 1.16, and 

 then put into clean water ; they are then to be transferred into an 

 acjueous solution, containing one-eighteenth of sal ammoniac. In 

 the mean time pure fine tin is to be fused, and copper added to it 

 in the proportion of 3 ounces to lOOlbs. of tin -, it is best to fuse 

 the copper with 6 ounces of tin first, and then add it to the rest. 

 When the tin is sufficiently heated, but not so highly as to prevent 

 its adhering to the iron, the weight is to be plunged into it, and 

 the tin will adhere perfectly. If the weights are to be polished, 

 they should be first turned in the lathe; when thus turned, they 

 are preserved from oxidation, are easily kept clean, and may be 

 used instead of copper weights. — Indiistriel. 



6. Temperature of the Planetary Space. — According to M. Fourier, 

 the tenjperature of the space occupied by our planetary system 

 is very nearly 40 octagesimal degrees colder than the tempera- 

 ture of fusing ice. — Annates de ChimiCj xxxvii. 309. 



7. Mr. Watt's Solar and Lunar Compasses. — Mr. Watt has 

 lately described, in the philosophical journals, what he calls solar 

 and lunar compasses. The following seems to be the last im- 

 provement. Stretch a circular disc of dark-coloured velvet of 

 about four inches diameter upon two very thin slips of light wood, 

 or upon two feathers placed across each other at right angles ; 

 render about 25 grains weight of pure filings of steel magnetic 

 by putting them between the folds of a piece of paper and 

 drawing the ends of two magnets about thirty times across them. 

 Rub the filings over the whole face of the velvet disc, they will 

 then sink into the spaces formed by the piles of the silk. Let 

 this be affixed to the end of a very light bar of wood, or to the 

 opaque part of a writing quill four inches long, by a fine needle 

 passed through the disc. Make a small perforation in the wood 

 or quill at the distance of one-third of its length, measuring from 

 the point at which the disc is attached; press a small agate or 

 glass capsule into the aperture without any wax or fixture ; the 

 elasticity of the wood or quill keeps it sufficiently firm ; balance 

 it on a fine steel point, and let a cover be put over it. This 

 instrument moves to the influence of the solar beam from morn- 

 ing to evening in our shortest days, even when the thermometer 

 stands at freezing, and though the rays fall upon it through the 

 glass of a window and the glass of the cover : the motion of 

 the balancing bar is as slow, equal, and constant, when the sky is 

 clear, as the shadow of the gnomon of a dial. Mr. Watt has also 

 observed, he says, that this instrument and several other bodies 

 clearly indicate by their motion the attractive influence of the 

 lunar beam. — Jamesons Journal^ 1828, p. 400. 



