156 ROUND THE YEAR 



diameter of 125, with an internal diameter of 75, the 

 area of cross-section would be the same in both cases. 

 But the hollow cylinder would have a power to resist 

 breaking greater than that of the solid cylinder in 

 the proportion of 17 to 10, while its resistance to 

 crushing would be more than twice as great. The 

 strongest tube is one whose external and internal 

 diameters bear the proportion of 1 1 to 5. 



Dr. Macalister shows that the cancellous or lattice- 

 work arrangement of the internal laminae of bones 

 coincides with the lines of pressure and tension. 

 Hence the bone is strengthened precisely where 

 strength is most needed, and the stresses are dis- 

 tributed. The bony substance is placed along the 

 curved pressure-lines and also along the curved 

 tension-lines, which intersect the first at right angles. 

 But the intervening neutral spaces, where there is 

 little or no thrust or pull, are left unoccupied, thus 

 economising material and diminishing weight as far 

 as possible. 



In some animals whose weight is " taken off" by 

 the water in which they live, increased weight of the 

 body is less disadvantageous, and economy of sub- 

 stance may be disregarded for the sake of additional 

 strength. The bones of a Crocodile are solid, and 

 composed almost to the core of a dense, ivory-like 

 substance. In a land-quadruped w r eight must be 

 more carefully considered, and the long bones are 

 largely excavated, the spaces being to a great extent 

 occupied by marrow. In a flying Vertebrate strength 

 and lightness are combined with still greater nicety 

 of calculation. The wall of the shaft is reduced to a 



