40 PEOSE HATJEUTICS. 



the sea that its waters may easily wash through^ and 

 never stagnate^ ' thus imitating the great main whence 

 they are derived, which never being of the same tempe- 

 rature, is in perpetual movement, and renewed every 

 hour.'* They may be made of tUes, ' opus signinum,' or 

 be excavated in the soHd rock ; in either case, in all 

 such ponds as are not perpetually motionless and asleep, 

 that extremity which lies furthest from the sea, and is 

 deeper and cooler than the other, should conduct by 

 straight or tortuous channels into a grotto where the 

 scaly troop may retire from the heat of the day, like 

 cattle, for refreshment and cover. The watery alleys 

 leading to these places of repose should not be too broad 

 for mursense, which prefer a narrow nestling trough; 

 some, however, object altogether to mixing mureense 

 with other stock, as they are liable to go mad like dogs, 

 and in that case will bite, run down, and destroy every 

 other species shut up in the same reservoir, till they 

 have entirely consumed them. In feeding these reser- 

 voirs, the supplies of water should be let in from one 

 side, and the issue, if possible, be made to take place at 

 the one opposite ; this will secure a perpetual renewal of 

 the water, which is a matter of prime importance here ; 

 a convenient coolness being also of equal consequence 

 for the salubrity of the fish, the deeper the source 

 whence the sea- water is procured, the better; and 

 wherever it is practicable the pond should fiU itself 

 from below. When the vivarium to be formed is 

 scarcely above the level of the sea, its basin should be 



* Columella's explanation of this phenomenon, ' quoniam ge- 

 lidnm ab imo fluctum revolvit pelagus in partem superiorem ' 

 (since the cold and deeper strata of water rise naturally to the 

 surface), is not true : the specific gravity of the water of the 

 Mediterranean not being below 40°, the point of greatest density 

 can have no tendency to the change here imputed to it : the 

 lightness of ice perhaps led him into this error. 



