264 THE DEPTHS OF THE SEA. [cHap. vr. 
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 
