The Philosophical magazine; a journal of theoretical, experimental and applied physics . -culum, not one was defective: &c. It the successful casting of these sixteen plates was due tothe base being of packed hoop-iron in place of a solid disc, itmust be admitted that Lord Rosses discovery was a greatimprovement upon the solid disc ; but if the ordinary prin-ciples of hydrostatics show that such an effect could not result,the cavities which were found on the surface of his first cast-ings upon a solid disc must be referred to some other cause;such as, that in his first attempts on the chilling
The Philosophical magazine; a journal of theoretical, experimental and applied physics . -culum, not one was defective: &c. It the successful casting of these sixteen plates was due tothe base being of packed hoop-iron in place of a solid disc, itmust be admitted that Lord Rosses discovery was a greatimprovement upon the solid disc ; but if the ordinary prin-ciples of hydrostatics show that such an effect could not result,the cavities which were found on the surface of his first cast-ings upon a solid disc must be referred to some other cause;such as, that in his first attempts on the chilling method,the melted metal not being sufficiently fluid for that mode ofcasting, it did not flow uniformly over the disc, but becomingsolid too quickly, left cavities in some places. If the metal retained its fluidity for ever so brief an intervalof time after an air-bubble was entangled between the ironand melted speculum metal, we can ascertain the nature andeffect of the forces acting on it by the principles of hydrosta-tics. Let AB be the upper surface of the chilling body, hac. a portion of air entangled within the melted metal the pressure at different points in a fluid increasing withthe depth, the parts at b and c will be subject to greater pres-sure than those about a; and hence the bubble of air will in-stantly take the form edf, and ascend as at hgk through thefluid metal, like air-bubbles in water, by virtue of the propertyof fluid pressure, that ?Hhe resultcmt pressure of a fluid^ on abody immersed in it, equals the voeight of the fluid displaced, andacts vertically upnoards through the centre of gravity of the fluiddisplaced. It is hence futile to prepare a way for escapedownwards when there is no tendency to escape in that di-rection. As fluids in contact, which do not mix, arrange themselvesin descending order according to their greater specific gravi-ties, it is impossible to see how air could place itself belowfluid metal; and we must conclude that the
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