264 



E. F. PIGOT. 



afterwards partially modified, and the whole weight of the 

 bob is carried by the tube T (fig. 1), which also serves to con- 

 tain the lamp and draw-tube for the lens, — the enveloping 



brass hemispheres being 

 thus rendered unneces- 

 sary. This modification 

 was suggested to me by 

 Mr. T. J. Murday, to 

 whom I am also indebted 

 for several other im- 

 provements which I have 

 much pleasure in acknow- 

 ledging. He has, at my 

 request, supplied the 

 following details of the 

 method adopted in mak- 

 ing the large heavy 

 hemispheres. These are 

 made of stereo-metal, 

 instead of lead, so as to 

 be workable in the lathe. 



Vertical Section of Pendulum-bob : 

 A, A, upper hemisphere ; B, B, lower 

 hemisphere; T, central supporting 

 tube ; L, glow-lamp. 



"The production of a true 

 sphere of relatively small 

 size is a fairly easy task in the hands of a skilled mechanic equipped 

 with proper tools ; but when the mass weighs 60 lbs. and measures 

 about 7 J inches in diameter, other methods are called for, and the 

 following brief details of the means adopted in this particular case 

 may not be out of place. From a hemispherical wood pattern two 

 castings in stereo metal were obtained, with the recess for the accu- 

 mulators cast in. Each section was carefully centred and drilled, 

 and afterwards mounted on a rigid steel mandrel, and the face 

 turned flat. The hemispheres were then mounted together on the 

 mandrel, — dowel pins having been inserted to keep them from 

 turning independently, — and the globe thus formed roughed into 

 shape. 



