138 PHYSICAL SCIENCE BK. in 



3 missions. So, too, in Macedonia, Theophrastus 

 asserts there is a river to which shepherds who 

 desire to turn their sheep white bring them. If 

 the sheep drink it for any length of time, their 

 colour changes as if they had been dyed. But if 

 those people want a dark wool, they have a dyer 

 ready at hand who charges nothing ; they have 

 merely to drive the same flock to the river Peneus. 

 I have recent authorities for the statement that 

 there is a river in Galatia that has the same 

 power of changing the colour in all animals, 

 while in Cappadocia there is one which if drunk 

 changes the colour of horses but not of any 

 other animal ; their skin is dappled with white 

 spots. 



4 It is well known that there are lakes whose 

 waters bear up those who cannot swim. There 

 used to be a pool in Sicily, there still is one 

 in Syria, in which brickbats float, and no objects 

 thrown in, however heavy, will sink. The cause 

 of it is obvious. Weigh any object and com- 

 pare it with water while they are equal bulk for 

 bulk. If the water is the heavier, it will bear the 

 object that is lighter than itself, and will raise it 

 above its surface to a height proportionate to its 

 lightness ; objects heavier than the water will sink 



5 in it. But if the weight of water and of the object 

 compared with it in respect of weight be equal, 

 the object will neither go to the bottom nor yet 

 will it stick up ; it will just be in equipoise with the 

 water. It will float, it is true, but almost submerged 

 and without any part projecting. The differences 

 in weight give the reason why some logs float 

 almost entirely above water, while some sink to 

 their centre, and some go down until they are in 



