260 The Rev. S. Haughton's Notes on Mineralogy. 



space below the piston is thus reduced absolutely to nothing, and 

 consequently the air contained in that space is completely ex- 

 pelled, however rarefied it may have been on entering from the 

 receiver ; and it follows that the ascent of the piston will again 

 leave a space perfectly empty, so that air will continue to flow 

 into it from the receiver so long as it has a pressure sufficient to 

 overcome its own inertia; and the action of the pump will con- 

 tinue perhaps for an indefinite number of strokes, so that the 

 pressure in the receiver must soon fall so low as to become inap- 

 preciable to our most delicate instruments. 



All the metallic parts of the pump should be made of iron, as 

 brass or bronze would be rapidly corroded by the oil. As has 

 been explained above, a slight leakage downwards takes place 

 through the piston at each ascending stroke : this circumstance 

 renders unnecessary that the piston should fit perfectly to the 

 cylinder ; so that a metallic fit without packing would be practi- 

 cally sufficient, as it is enough that the leakage during one stroke 

 do not exceed the volume of the tubular portion of the piston. 

 In order to be able to take the machine asunder, there should be 

 a screw joint in the valve-rod s z close to the valve. When the 

 cover is removed, the upper portion of the piston-rod a un- 

 screwed, and the piston brought to the bottom of its stroke, the 

 nut on the top of the piston q may be grasped by pincers and 

 the rod turned so as to unscrew the joint above mentioned. The 

 lower portion of the valve-rod is prevented from turning by the 

 tail of the valve being square in section and fitted into a guide of 

 similar shape. Once the small piston is removed, nothing hin- 

 ders the removal of the principal piston and of the cylinder, and 

 then the valve v remains free to be examined or cleaned. The 

 joints above the piston do not properly form part of the pump, 

 and the cover is adopted merely to prevent the atmosphere from 

 pressing upon the piston; it is therefore unnecessary to have 

 these joints covered with oil. 



Palermo, September 1, 1866. 



XXXIII. Notes on Mineralogy. By the Rev. Samuel Haughton, 



M.D.j Fellow of Trinity College, Dublin. 



[Continued from p. 227.] 



No. XVIII. On the Meteoric Stone that fell at Dundrum in the 



co. Tipper ary, on the \2th of August, 1865*. 



THE meteoric stone that forms the subject of the present 

 paper, fell near Dundrum, co. Tipperary, under circum- 

 stances that were described to me as follows, by the man in 

 whose garden it fell : — 



* Read before the Royal Irish Academy, 1866. 



