214 SEASON 1875-76. 



instincts, and nibble the tins of their brethren and sisters. This 

 may be mitigated in some way by filling the pond with 

 stagnant water for three weeks before the fry are placed in it ; 

 but the growth of crustacece is so dependent on the weather, much 

 reliance should not be put in this method. 



The depth thus being fixed, the direction of the pond must be 

 considered. In Scotland, it is found that fry do much better in a 

 pond which runs north and south than in one running east and 

 west. The reason is that the east wind is the most deadly foe 

 they have to contend with, and that of course, if the pond runs 

 with the wind, it gets much more of it than if it runs across it. 

 But narrow ponds, lying north and south, also keep much cooler 

 than those running east and west, the reason being that if a pond 

 exposes its side to the sun, the reflection heats the water more 

 than if the end only reflected. Of course the surface of the water 

 in a pond running north and south gets more sun than the surface 

 of the water in a pond running east and west ; but the reflection 

 from the sloping sides is so important a factor that it far more 

 than counterbalances it. Heat-rays, whether obscure or luminous, 

 are refracted in the same way as rays of light when they enter a 

 diathermanous medium. (Ice is only diatherrnanous in regard to 

 the luminous rays, it arrests the rays of obscure heat.) It is 

 the sun's rays we have to consider in connection with placing 

 longer axis of rectangular ponds. The slope of the sides is usually 

 between 45 and 60. The refraction of the water has the effect 

 of making the heat-rays strike the side nearer the bottom than if 

 the path of the ray was straight. The slope above the water also, 

 when the pond runs east and west, reflects the rays back on to 

 the surface of the water, and these rays are also refracted. The 

 consequence is that extremes of temperature are produced in a 

 pond running east and west, and this in itself is fatal to good 

 work in trout-rearing, to say nothing of the danger of the limit 

 of the temperature being reached unexpectedly. 



