THE 'FROG.' 35 



had been anticipated in nature — not only for many- 

 years before carriages were invented, but many ages 

 before man could have existed. 



The next object is to prevent the horse from 

 slipping on wet or smooth surfaces. For this object 

 the frog is wonderfully adapted, as it will cling to 

 ice or a wet boulder, and enable the animal to 

 traverse a slippery surface with perfect freedom. 



When, therefore, the hoof of an unshod horse 

 comes to the ground, and the weight of the animal 

 rests upon it, the hoof has a double hold, the frog in 



LTYJ 



CENTRAL BEAKING OF RAILWAY CARRIAGE. 



the centre clinging like indiarubber, and the sharp- 

 edged wall holding to the least roughness or irregu- 

 larity. 



Like the wall, the frog is subject to perpetual 

 wear, and therefore must be perpetually renewed. 

 It is not, however, rubbed down by friction, as is 

 the wall, but its outer portions continually become 

 detached in little loose flaps, which hang on for a 

 time and then break away altogether, so as to make 

 way for the fresh material which has been formed 

 above them. 



d2 



