448 RED EARTHENWARE POISONOUS. COMMON RED EARTHENWARE It is not so generally known as it should be, that the glaze of the common red earthenware, in which the food of the lower classes is mostly prepared, is composed of lead, which is in this state slightly soluble in animal oil, and more copiously in the acids of our common fruits, especially when their action is assisted by the heat necessary for cooking these articles. Many of the obscure visceral diseases of the poorer classes are to be attributed to this little-suspected source; and the temporary removal of the pain occasioned by them is one of the motives which leads to the habitual use of spirituous liquors. A Mr. Meigh, two or three years ago, received a premium from the Society of Arts for a glaze which is perfectly free of any ingredients noxious to health; being composed of one part Cornish granite, one part glass, and one part black manganese, the whole well ground, and dif fused in water to the consistence of cream. But the Staffordshire potters are, like other people, so attached to old customs, that the poisonous glaze of lead is still that which they generally make use of, to the great injury of the health of millions. Z. COLOURS MECHANICALLY AFFECTED Lewis observes, of black colours, in his "Commercium Philosophico-Tech nicum," that" one and the same body will assume different degrees of blackness, according to the disposition of the sensible parts of its surface. Thus, black velvet, when the pile is raised, appears intensely black, much more so than the silk it was made from; but on pressing the pile smooth, it looks pale, and in certain positions shews somewhat even of a whitish cast." These changes áre readily accounted for, by the theory which ascribes the blackness of bodies to the luminous rays that fall upon them being a great part absorbed in their When the surface is compores. posed of a number of loose filaments, or small points, with their extremities towards the eye, much of the light is lost in the interstices between them, and the body appears dark: when these filaments are pressed close, or the surface smoothed and polished, more of the light is reflected from it, and the intensity of the blackness is diminished. MINOR CORRESPONDENCE. Sir, -In common with many of your readers, I feel great pleasure in finding that we are at length to be favoured with a description of Mr. Child's much-talked-of model. Having myself invented a machine for drawing spirals, the principles of which I have explained to several of my friends, and, amongst others, to Mr. Christie, of the Mechanics' Institution, and Mr. Christopher Davy, architect, I beg to apprise you of the same before the appearance of Mr. Child's machine, in order that I may not be charged with plagiarism should there be any similarity in our inventions. I hope shortly to have the pleasure of forwarding to you a description of my method, as also of two methods of describing a cycloid; the one by means of rack and pinion, and the other by sliding and circular motion. I remain, Sir, Your most obedient Servant, J. MURDOCH, Mec. Draftsman. 4, Victoria Place, Mile End Road, Thursday, Aug. 13. [The preceding letter was sent to us on Friday, the 14th of August, by Mr. Davy, who informs us that it was put into his hands for transmission on the day preceding. ED.] INTERIM NOTICES. Part LXXIX. for August, and the Supplement to Vol. XI, containing Title, Preface, and Inder, with a Portrait of Dr. Olinthus Gregory, will be ready on the 1st September, price 1s. 6d. Also, the Supplement separately, price 6d. and Vol. XI. complete, in extra boards, price 88. The letter of "Fair Play," on the rival inventions of Messrs. Ibbetson and Child, came too late to hand for insertion this week. We admit, in the meanwhile, that the terms of praise in which we spoke of the specimens sent us by Mr. Child's friend, and published in our last Number, require some little qualification. On examining them more narrowly than we had time to do last week, there does appear to be a greater inequality in the dividing loops than we at first sight supposed to be the case, or have been able to detect in many of Mr. Ibbetson's productions. "J. Turner" is referred to the article in our last Number, on the "Hardening of Thin Steel Plates," for a proof that the thing is not as he affirms-" impossible." We shall be obliged to S. P. W for a continuation of his. "Notes." Communications .received. from G. S.- Mr. Baddeley-L. P.-W..D.-A Mining CaptainQuerist Z.-Mr. Henshaw Minus K. 1. G. Published, for the Proprietor, by M. SALMON, at the Mechanics' Magazine" Office, No. 115, Fleet Street; where Communications for the Editor (post-paid) are requested to be addressed. J. MOYES, Took's Court, Chancery Lane, END OF VOL. XI, INDEX TO VOL. XI. Baddeley, Mr. Wm. on fire-engines, 6, Bailly, Mr. F. on the Nautical Alma- Barnard, Mr. J. on fire-escapes, 66, Barton's steam-piston, 71 Baverstock, Mr. John, mathematical Bayley, Mr. G. account by, of the rais- Beer, potato, 127 Bell founding, improvement in, 376 Blakey's anchor stopper, 407 Bread, new species of, 31 Bridge, Delafons's suspension, 68 Brigg's new steam-engine, 356 Bright's shadowless lamp, 68 British Almanac, 409 Bronzing medallions, 437 Bullman's washing-machine, 94 Burgess's new carriage, 413 Burney, Dr. on meteorological diaries, Buston's, fire-escape, 82 C. Calendar of the Memory, Snookes', Calico printing, antiquity of, 101 Cement, powerful, 191 INDEX. Chapman, Mr. Thos. on an improve- Chemical enquiries, 82; answered, 272, Child's, Mr. turning apparatus, 57, 105, parabolic trammel, 241 cowl, new, 168 Chiverton, Mr. Benj. on paddle-wheels, Church bells, improvement in founding, Church clocks, on, 34, 117, 262, 323, Climbing-boy system, 80, 96, 151, 160 Coach-wheels, proper size for, 304 Coinage, ancient mode of, 101 Colours mechanically affected, 448 Combustion, spontaneous, 123, 352 Copernican system, objections to the, Curvilineal spaces, on finding the area Cycloid, on the, 91, 142, 219, 233, D. D- Henry, plan by, of a skater's Davies's, D. fire-escape, 66, 101; hint adaptation of the overture of Oberon apparatus for laying moor- Dodd, Lieut. H. W. on vertical paddle- Donaldson, Mr. James, on flooring Don's metallic blinds, 123 Falling bodies, on the laws of, 323, Farey, Mr. Jos. on the power of steam- Fire, Society for the Prevention of the plan by Mr. Wm. Russel, engi- engines, on, 6; improvements in, 49 INDEX. Barnard's, 66, 458; Buston's, 82; 430 balls, instructions for observing, Gas, from rosin, 127; from wool re- plan for employing, to heat steain- inflammable, inspiration of, 384; hydrogen, apparatus for collect- Gases, hints on the production of the, Geometrical questions, answers to, 120 Gibson, Mr. W. account by, of Blakey's Gill, Mr. on the French process for Glass, American improvement in the Grain, how to correct damaged, 101 his Mathematics for Practical Men, 72 Groins, new method of finding the curve G. S. answer by, to Mr. Foord, on find- Gun-locks, important improvement in, H. Hail, cause of, during warm weather, Hall, Mr. Jos. on a property of num- Hardy's inverted spring pendulum, 68 Heavens, blue appearance of the, ex- Hebert's boring apparatus, 68 Heights, how to measure by shadows, Hesse's fire-escape, 66 Higginson's, Lieut., improvements on Hogan, Mr. Thos. demonstration by, of Hopwood's, Mr. H. chemical enquiries, Hot atmospheres, working in, 176 Hughes, Mr. Thos. mathematical solu- I. Ibbetson's, Mr. geometric chuck; far- J. INDEX. Manchester Society for Promoting Use- Mason, Mr. Wm. on the Copernican Spitalfields, 239 Medical spoon, Gibson's, 94 Meteorological observations made at Epping, from 1821 to 1829, 218 diaries, on the common mode of keeping, 218, 379 Meteors, Dr. Maskelyne's instructions Microscopic specimens, Dr. Goring's, Mill spindles, English and American, Mill-work, case in, 22 Miners, questions to, 12; answered, 271 Moon, influence of the, 426 Motion, new mechanical, 1, 168 -, on the production of, in bodies Music, on, and temperament scales, 291 |