554 Progress in Science. [0(5tober, 



best group of two children under six years of age ; ioo dols. for the best land- 

 scape. The proofs to be i6£ by 2i£ centimetres, mounted on cards 25^ by 

 3o£ centimetres. 



At the same meeting M. Champion gave the following as the result of his 

 experiments on the preparation of gun-cotton : — The acid mixture consists 

 of 2 measures of nitric acid at 40 B., obtained by mixing common and 

 fuming nitric acids, 3 measures of sulphuric acid at 66°. The mixture may 

 be used either cold or at 40 C. The cotton is left in contact with the acid 

 for three minutes, and the product washed till perfectly neutral. 



Dr. E. Priwoznik records a change in cast-iron produced by the adtion of a 

 mineral sulphur water. On examining an iron water-pipe which had been ex- 

 posed for twelve years to the action of water rich in the sulphide of hydrogen, 

 the innermost stratum was found to consist of — 



Hydrated oxide of iron 8ro8 



Free sulphur 12-29 



Sulphide of iron 4*48 



Hygroscopic water 0*57 



Nickel, cobalt, magnesia, silicic acid (soluble and] 



insoluble), traces of carbon, and chlorides of am- 1 1*58 

 monium and sodium ) 



ioo-oo 

 This stratum is, therefore, an intimate mixture of hydrated oxide of iron, 

 sulphide of iron, and sulphur. The hydrated oxide has the composition 

 2Fe 2 3 ,3HO, and is therefore identical with limonite. The middle stratum 

 contained 79-2 per cent of metallic iron, and the exterior 92-6. 



Interesting researches on the stroboscopic determination of the pitch of 

 tones have been made by M. Mach. In the apparatus there is a cylinder 

 which makes three revolutions in a second, and is divided into five octaves, 

 At one end of it begins 10 bands, which, however, become more numerous and 

 dense towards the other end, being there 320. To the axis of a syren is fixed 

 a disc having equidistant radial slits of the same number as the holes in the 

 syren-disc. The surface of the rotating cylinder is looked at through this 

 slitted disc, while the syren tone is gradually raised. According to the stro- 

 boscopic principle the bands look distinct and at rest where there pass before 

 the eye an equal number of them and of slits in the disc. If a scale of num- 

 bers of vibration be attached to the cylinder, the number of vibrations of the 

 syren can be at once ascertained by observing the part corresponding to the 

 distinct and still ring of the cylinder. One sees, however, distinct and at rest, 

 not only the part of the cylinder corresponding to the number of vibrations 

 of the syren, but also all those parts which correspond to the harmonic over 

 tones. Of all such parts it is, of course, that one which furnishes the smallest 

 number of vibrations that corresponds to the vibration-number of the syren. 

 The determination may be varied in accuracy by varying the bands on the 

 paper of the rotating cylinder. The apparatus may be applied to other 

 sounding bodies. Thus let a mono-chord string be stretched at right angles 

 to the axis of the cylinder ; then simple teeth (Zachen) appear where the 

 sounding string is opposite that part of the cylinder indicating the same 

 number of vibrations. Another application is to attach small mirrors to 

 tuning-forks, and watch in them the image of the rotating cylinder. An organ 

 pipe may be also submitted to observation with aid of Konig's capsules and 

 dancing jets. 



Errata. — Page 474, footnote, for plane read slane. Page 480, line 22 from 

 bottom, for bog read boy. Page 540, line 25 from bottom, for north-western 

 read south-western. 



