1894.] on the Newtonian Constant of Gravitation. 363 



pressed home upon its shoulder. The lower half of each mould has 

 a |-inch central cylindrical hole, into which the lug of the brass ball 

 holder exactly fits. There is also a small hole at the side which can 

 be stopped with a brass plug. The balls are made as follows : — The 

 interior of the mould is smoked and then screwed up as tight as 

 possible. It is then heated until a piece of lead upon it begins to 

 melt. The necessary quantity of pure lead melted in an earthen pot 

 is then carefully skimmed and poured in until the cylindrical neck is 

 full. The mould is then made to rest upon a cold iron slab, and a 

 large blowpipe flame is directed upon the upper part so that it cools 

 from below upwards, and not from the surface inwards ; more lead is 

 added to keep the neck full. As soon as the lead in the neck solidifies 

 the plunger is inserted, and the whole is placed in a hydraulic press. 

 The plunger is forced down upon its seat ; the lead, already free from 

 bubbles and vacuous cavities, is compressed until at last tbe excess of 

 solid metal flows through the small side hole in the form of wire. 

 The ball is thus made true in form, necessarily homogeneous, which 

 no alloy is likely to be, and definite in size. When cold it can be 

 lifted from the mould, then, after cutting off the wire which projects 

 from its equator, it is ready for weighing. 



The small gold balls are made by melting the required quantity 

 of pure gold in a hole in a bath brick, and, as in the case of the lead, 

 letting it cool from below upwards, so as to avoid cavities. It is then 

 inserted in a pair of polished hemispherical hardened steel dies, which 

 Mr. Colebrook made for the purpose, and beaten, being turned between 

 each blow, and annealed once or twice until a perfect polished sphere, 

 without a mark upon it, is the result. I make these in pairs of exactly 

 the same weight, and, as in the case of the lead balls, thus obtain truth 

 of form, accuracy of size and homogeneity, all in a very perfect — more 

 than sufficiently perfect — degree. These are each suspended from a 

 quartz fibre of the necessary length, to the other end of which a book 

 and eye is fastened. Into the very important details of these opera- 

 tions it is impossible, for want of time, for me to enter. The gold balls 

 are • 2 and ■ 25 inch in diameter, and a pair of gold cylinders were made 

 in a similar tool ■ 25 inch in diameter, and about the same length. 



Perhaps the most important detail in the whole apparatus is the 

 " beam mirror," which is of the form shown in Fig. 3. It is necessary, 

 as far as possible, to reconcile the following incompatible conditions. 

 It should be as light as possible, have as small a moment of inertia as 

 possible, the optical definition should be as perfect as possible, and, 

 almost most important of all, the form should be such that the resist- 

 ance offered by the viscosity of the air should be reduced to the 

 smallest possible degree. By cutting the middle portion out of an 

 optically perfect round mirror all these conditions are realised in 

 some degree, and the optical definition is actually more perfect in the 

 horizontal direction than that due to the whole disc. This is fastened 

 to a cross-shaped support of gilt copper. The ends of the mirror 

 have vertical grooves of microscopic fineness cut in their thickness, so 



