264 THE DEPTHS OF THE SEJ. [CHAP, vi 



upon the same spot, with like success. The deptl 

 in fathoms should be carefully noted, as a most im 

 portant element in determining the conditions of lift 

 and distribution of species; and the nature of the 

 bottom whether mud, sand, or gravel; and if th( 

 latter, it is well to state the nature and compositior 

 of the pebbles, and if possible the source from whicl 

 they may probably have been derived. Now tha 

 we have in the Miller- Casella thermometer a reliabL 

 instrument for this purpose, the bottom temperaturt 

 ought always to be noted. This is important whethei 

 in shallow or in deep water. In shallow water 

 gives a datum for determining the range of annual 

 variation of temperature which can be endured 

 certain species ; and at great depths it is even more 

 important, as we are now aware that, owing to the 

 movement of masses of water at different tempera 

 tures in various directions, totally different condi 

 tions of climate may exist in deep water within a 

 few miles of one another, and the limits of thest 

 conditions can only be determined by direct experi 

 ment. It is important when determining the bottom 

 temperature to note also the temperature of the 

 surface of the sea, the temperature of the air, the 

 direction and force of the wind, and the general 

 atmospheric conditions. If the dredger be purely a 

 zoologist, having no particular interest in special 

 physical problems, it will still be well worth his 

 while to make all the observations indicated and to 

 publish the results. These then pass into th 

 hands of physical geographers, to whom all trust 

 worthy additions to the myriad of data which ar 

 required to arrive at a true generalization of th 



