264 THE DEPTHS OF THE SEJ. [CHAP. vi. 



upon the same spot, with like success. The depth 

 in fathoms should be carefully noted, as a most im- 

 portant element in determining the conditions of life 

 and distribution of species; and the nature of the 

 bottom whether mud, sand, or gravel; and if the 

 latter, it is well to state the nature and composition 

 of the pebbles, and if possible the source from which 

 they may probably have been derived. Now that 

 we have in the Miller-Casella thermometer a reliable 

 instrument for this purpose, the bottom temperature 

 ought always to be noted. This is important whether 

 in shallow or in deep water. In shallow water it 

 gives a datum for determining the range of annual 

 variation of temperature which can be endured by 

 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 these 

 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 the 

 hands of physical geographers, to whom all trust- 

 worthy additions to the myriad of data which are 

 required to arrive at a true generalization of the 



