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 LXII. Intelligence and Miscellaneous Articles. 



ON BAROMETRIC COMPENSATION OP THE PENDULUM. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



REFERRING to Professor Heller's article "On a Barometer 

 without Mercury " in the last Number of the Philosophical 

 Magazine, page 401 &c, I may mention (as another possible adap- 

 tation of the same fundamental principle to a different purpose and 

 in a different form) a plan which occurred to me some time since for 

 correcting the barometric inequality of the ordinary clock-pendulum, 

 by affixing to the pendulum-rod, produced upwards, a body of the 

 same volume as the pendulum-bob, but of very small weight. This 

 implies that the upper part of the pendulum-rod has the same length 

 as the lower part ; if it be shorter, the volume of the light body must 

 be larger. 



It appeared to me, however, probable that, in practical use, incon- 

 venience might be produced by this construction, and I never actu- 

 ally made a trial of it. 



Royal Observatory, Greenwich, "• "• Airy. 



May 3, 1871. 



ON THE USE OF THE SPECTRAL APPARATUS IN THE QUANTITA- 

 TIVE DETERMINATION OF COLOURING-MATTERS. BY K. 

 VIERORDT. 



In the spectral apparatus hitherto in use the slit is bounded by 

 two plates, a fixed and a moveable one. For the present purpose 

 the moveable plate is divided into two parts, an upper and a lower 

 one. Each of these plates is provided with a fine micrometer-screw, 

 by means of the milled head of which the breadth of the slit may be 

 accurately determined. If both the halves, the upper and the lower, 

 are of exactly the same width, the upper and the lower half of the 

 spectral field of view is of exactly the same intensity in every part. 

 But if a transparent coloured medium is brought in front of the 

 upper slit (a coloured glass for instance, a thin lamina of a coloured 

 body, a coloured organic structure, or, finally, in a small glass trough 

 with parallel sides a solution of any coloured substance or the co- 

 lourless solution of a strongly fluorescent body), the spectrum is 

 divided into two halves, one above the other, of different luminous 

 intensities — into the pure spectrum of the source of light produced 

 by the part which remains free, and into the spectrum modified by 

 the interposed coloured body. 



I may mention another accessory arrangement which I have intro- 

 duced into the spectrum-apparatus, which I can also recommend for 

 other spectrum-analysis purposes than the present, namely an ar- 

 rangement in the eyepiece of the observing-telescope which renders 

 it possible to stop all parts of the spectrum excepting that investi- 

 gated. By this arrangement very feeble coloured lines of the spec- 

 trum of chemical elements may be observed with great accuracy. 

 For details I must refer to my work, which has just been published 



