CH. V] MECHANICAL RECREATIONS 93 



Columbus's Egg. The toy known as Columbus's egg depends 

 on the same principle as the magic bottle, though it leads to 

 the converse result. The shell of the egg is made of tin and 

 cannot be opened. Inside it and fastened to its base is a 

 hollow truncated tin cone, and there is also a loose marble 

 inside the shell. If the egg is held properly, the marble runs 

 inside the cone and the egg will stand on its base, but so long 

 as the marble is outside the cone, the egg cannot be made to 

 stand on its base. 



Cones running up hill*. Another common experiment, 

 which produces the optical effect of a body moving by itself up 

 an inclined plane, also depends on the tendency of a body to 

 take a position so that its centre of gravity is as low as possible. 

 Usually the experiment is performed as follows. Arrange two 

 sticks in the shape of a V, with the apex on a table and 

 the two upper ends resting on the top edge of a book placed 

 on the table. Take two equal cones fixed base to base, and place 

 them with the curved surfaces resting on the sticks near the 

 apex of the V, the common axis of the cones being horizontal 

 and parallel to the edge of the book. Then, if properly 

 arranged, the cones will run up the plane formed by the sticks. 



The explanation is obvious. The centre of gravity of the 

 cones moves in the vertical plane midway between the two 

 sticks and it occupies a lower position as the points of contact 

 on the sticks get farther apart. Hence as the cone rolls up 

 the sticks its centre of gravity descends. 



Test of Internal Structure. Here is another simple experi- 

 ment of a somewhat different character. Suppose two balls 

 constructed of equal size and weight, one of lead and hollow in 

 the middle, the other of copper and solid ; and suppose that 

 both spheres are gilt so that in weight, appearance, and elasti- 

 city they are indistinguishable. How can we tell which of 

 them is solid ? The answer is by allowing them to roll down 

 a rough inclined plane, side by side. 



Perpetual Motion. The idea of making a machine which 

 once set going would continue to go for ever by itself has been 

 * Ozanain, 1803 edition, vol. n, p. 49 j 1840 edition, p. 216. 



