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2538. Causes of Want of Accuracy. The use of an elastic fruit in early experiments leads to a want of accuracy in the first, steps of the operation, which causes very annoying difficulties afterwards; and therefore a solid, inelastic body--an egg boiled hard--is recommended as the first object to be imitated. 2539. Making the Moulds--First Half. Having filled a small pudding basin about three-quarters full of damp sand (the finer the better), lay the egg lengthways in the sand, so that half of it is above, and half below, the level of the sand, which should be perfectly smooth around it. Then prepare the plaster in another basin, which should be half full of water. Sprinkle the plaster in quickly till it comes to the top of the water, and then, having stirred it for a moment with a spoon, pour the whole upon the egg in the other basin. 2540. Making the Moulds--Second Half. While the half mould thus made is hardening thoroughly, carefully remove every particle of plaster from the basin in which it was mixed, and also from the spoon which has been used. This must be done by placing them both in water and wiping them perfectly clean. This is highly important, since a small quantity of plaster which has set will destroy the quality of a second mixing if it is incorporated therewith. In about five minutes the half mould will be fit to remove, which may be done by turning the basin up with the right hand (taking care not to lose the sand), so that the mould falls into the left hand.