NEWTONIAN CONSTANT OF GRAVITATION. 27 
truly that the steel disc used as a template would audibly rattle when placed in 
either alone, but could not be got in at all when a single strip of cigarette paper was 
inserted on one side only. The two halves can be screwed together by means of six 
steel bolts as shown. A 4-inch hole is bored in the centre of one hemisphere and 
one of 3-inch in the centre of the other. Into the latter an accurately fitting steel 
plunger was inserted, and when pressed down to the head, was turned at the inner 
end, so as to complete the sphere. A small hole is drilled on the equator, enlarged 
almost immediately to a greater size. Into this a brass plug can be pushed. Before 
being used, the mould is warmed, and the internal surface smoked with a gas flame. 
Into the 1-inch hole the brass ball-holder e is inserted. A number of these were made 
by Mr. Conesrook, of the utmost possible accuracy of the form shown, this being a 
4-inch sphere, surmounted by a + X 4-inch cylinder with a shoulder of ; inch, of 
such a depth that when pressed home the $-inch sphere should be tangential to the 
41-inch sphere. Though these ball-holders were made to measure only, their weights 
were closely alike, being 10°70 grams for each before cutting the slot and drilling the 
eross hole, and 10°28 afterwards. Since the whole effect of the gravitation of these 
ball-holders and of the 44-inch lead balls, all in their ultimate positions, is 7 yp in 
excess of what it would be on the supposition that the whole mass is concentrated at 
the centres of the lead balls, any doubt as to the amount of the correction, which 
cannot be so much as one part in a hundred, leaves an uncertainty in the result of 
about one part in a million, and is of no consequence. I should state here that this 
correction includes the very smali pieces of brass fastened to the lower end of the 
wires, which with their pins were made to occupy the same volume as the material 
removed in the slot and cross hole. The brass ball-holder, before being inserted 
in the smoked mould, was tinned on the spherical surface, and then wiped to remove 
superfluous metal. The mould was then put together, and the steel bolts, after being 
well rubbed with blacklead, screwed up as tightly as possible. The mould was then 
slowly heated over a Fletcher gas burner, until a piece of lead lying upon it began to 
melt. The brass plug was then inserted in the side hole, and pure skimmed lead 
was gently poured in through the neck from an earthen pot until it was full. The 
mould was then lifted on to a cold block of iron, but a large blow-pipe was kept 
playing on the top of it, the effect of which was that the metal slowly solidified from 
below upwards. The progress coald be followed by inserting a fine carbon rod, or 
more evidently by watching the contraction of the metal in the neck. It was 
necessary to add lead from time to time to keep the neck full, and in the case of the 
41-inch ball the amount required would have filled about 3 inches of the neck had 
it been so long. In this way perfectly sound castings, free from vacuous cavities, 
which always form when the metal solidifies on the surface first, are easily obtained ; 
but to make the metal free from pores, and to close up any such cavities should they 
by any possibility exist, the moulds were placed in the hydraulic press immediately 
the metal in the neck had become solid, and after removing the brass plug, the steel 
E 2 
