ON THE BAROMETER. 



43 



sibility would then allow the silver to be 

 poured round it in a fluid state, without 

 injuring the texture of the platina. 



A cylindrical mould of tlie third of an 

 inch in diameter was made; in the centre 

 of which, wa sfixed a platina wire -j^^ of 

 an inch in diameter, and the intervening 

 space was filled with melted silver. When 

 this rod was drawn to j^ the platina 

 which it contained in its centre was reduc- 

 ed to y^^^oj ^"^1 by a successive reduction, 

 and application of nitrous acid to dissolve 

 away the external coat of silver, platina 

 wires of ^j^Vo' ^"*^^ r^iv were obtained 

 which are very useful for applying to eye 

 pieces of astronomical instruments, and 

 are as fine as can be required for such pur- 

 poses, since in a thirty inch telescope, 

 TToa °^ ^" ^"<^'^ subtends only the second 

 of a degree. 



The most convenient method of mana- 

 ging the removal of the silver from these 

 delicate wires, is to bend a portion into 

 the shape of the letter U, with small hooks 

 at its upper extremities, by which it may 

 be suspended and immersed in the nitrous 

 acid, and the silver will then be dissolved 

 away from all the part between the hooks, 

 leaving the hooks themselves as a means 

 of conveying and attaching the wire when- 

 ever wanted. The platina wire is suffici- 

 ently visible to the naked eye, when the 

 diameter does not exceed ^^Vo" °^ ^" inch, 

 but when much less it eludes the sight, and 

 the slightest breath of air would carry it 

 away, if it were not retained by the ends 

 that still retain the silver. It is of conse- 

 quence to have the platina in as dense and 

 solid a state as possible before it is drawn 

 out, and for this purpose, Dr. W. first 

 brings a considerable globule of it into 

 perfect fusion, under the flame of a spirit 

 lamp, impelled by a blow-pipe fed with 

 oxygen gas, (which was first suggest- 

 ed by Dr. Marcet,) after which, the 

 globule is hammered out, and wire- 

 drawn, before it is coated with silver. 

 Dr. W. has succeeded in obtaining; small 

 portions of platina wire, which by calcu- 

 lation, could not exceed 3-0 Voo P^^t of an 

 inch in diameter, but at ^^q-^ of an inch, 

 though quite invisible to the naked eye, 

 it would support 1§ of a grain before 

 breaking. — Retrospect. 



ON THE BAROMETER. 

 By Richard Walker, Esqr. Phil. Mag. No. 172, 



The Barometer when stationary, and 

 Iiaving a concave surface^ at the top, 

 is almost an infallible indication of rain at 

 the place of observation, or in its vicinity, 

 especially if the barometer be at, or below 

 changeable. The barometer when sta- 

 tionary, and with a convex surface of 

 the mercury at the top, is a strong indi- 

 cation oi fair weather, at the place of ob- 

 servation, or in its vicinity; especially if 

 the barometer be at or above changeable. 

 In the first instance, Mr. Walker accounts 

 for the circumstance, by the natural spring 

 or elasticity of the air, being suspended, 

 or diminished, (the density of the at- 

 mosphere remaining the same,) by the in- 

 tervention of vapour collecting into a 

 mass, and in the latter instance, to the dis- 

 persion of the interposed vapour, by which 

 the natural elasticity of air is restored. 



For the same reason it is he presumes, 

 that during a steady fall of rain, the ba- 

 rometer is commonly stationary, with a 

 concave surface at the top, and as soon as 

 the 'weather begins to clear up, the top 

 of the mercury in the barometer as- 

 sume a convex surface. The quick as- 

 cent of smoke, vapour, &c. into the at- 

 mosphere, indicates fair weather; and the 

 slow ascent, and particularly the descent 

 of them indicates rain. The former in 

 consequence of an influx of denser, or 

 heavier air, into the lower stratum of the 

 atmosphere, commonly proceeding from 

 the north and east points. And the lat- 

 ter, in consequence of an influx of rarer, 

 or lighter air, into the lower stratum of 

 the atmosphere, proceeding commonly 

 from the south and west points. 



Upon the whole, there is more wet 

 weather, while the barometer is above 

 changeable, than there is dry, while the 

 l)arometer is below changeable. Hence 

 it follows, that the former state of the 

 barometer is not so strong an indication 

 of fair weather, as the latter is of rainy 

 weather. — Retrospect. 



On THE PRESERVATION OF ZOOLOGICAL 



Specimens. 



By M.^1. J'ei'on & Le Sueur, Jour, de Phys. Oct. 1 810. 



Glass upon the whole combines more 



