1* On thg casting qfSpeailafor Tteflectitig Telescopei. 



expedient, that of sixteen plates cast for the three-feet spe- 

 culum, not one was defective:" &c. 



If the successful casting of these sixteen plates was due to 

 the base being of packed hoop-iron in place of a solid disc, it 

 must be admitted that Lord Rosse's discovery was a great 

 improvement 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 of 

 casting, it did not flow uniformly over the disc, but becoming 

 solid too quickly, left cavities in some places. 



If the metal retained its fluidity for ever so brief an interval 

 of time after an air-bubble was entangled between the iron 

 and melted speculum metal, we can ascertain the nature and 

 effect of the forces acting on it by the principles of hydrosta- 

 tics. Let AB be the upper surface of the chilling body, bac 



a portion of air entangled within the melted metal ABCD. 

 Now the pressure at different points in a fluid increasing with 

 the 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 the 

 fluid metal, like air-bubbles in water, by virtue of the property 

 of fluid pressure, that ^^the resultant jjressure of a Jluid, on a 

 bodj/ immersed iti it^ equals the weig/it q/' t/ie Jluid displaced, and 

 acts vertically upwards through the centre of gravity of the Jluid 

 displaced." It is hence futile to prepare a way for escape 

 downwards when there is no tendency to escape in that di- 

 rection. 



As fluids in contact, which do not mix, arrange themselves 

 in descending order according to their greater specific gravi- 

 ties, it is impossible to see how air could place itself below 

 Jluid metal; and we must conclude that the presence of cavi- 

 ties on the face of his castings indicate that his Lordship's 

 metal was then deficient in fluidity, and, flowing unequally 

 over the chilling surface, on becoming too quickly solidified, 

 contained the cavities which were found. 



