OF THE ATLANTIC OCEAN. 239 



the standard thermometer at Kew have been applied. 

 Unhickily soon after entering the Bay of Biscay on the 

 return voyage, and when the observations (which were 

 undertaken chiefly for the purpose of ascertaining whether 

 the system of currents known to exist in that Bay could 

 be ascertained by the temperature of the water) were be- 

 coming most interesting, the thermometer was broken, 

 and the return series is thus incomplete. 



On the run out the gradual deepening of the water is 

 the first thing which tells notably on the thermometer. 

 Indeed the delicacy of the action of the instrument at 

 depths of from 80 to 100 fathoms and upwards is sur- 

 prising. On referring to the table of observations it will 

 be found that the thermometer, showing the temperature of 

 the sea water, fell on the nth September from 65" i to 64, 

 although the ship was standing to the southward at the 

 time, and no land was in sight. On examining the chart 

 it was found that the depth had decreased from 130 

 fathoms to 80. Hence the fall. 



I cannot account for the two observations made at noon 

 and at 9 p.m. on the <^th September being identical, espe- 

 cially as the temperature of the air was not the same in 

 both cases, and the second observation was taken nearly 

 ninety miles S.W. by S. from the first, except by sup- 

 posing the ship to have passed twice through a cold 

 current at two points of its course. 



The influence of the shoaling of the water is clearly 

 shown in the first observation on the 12th September, 

 which was taken inside the Burliugs, and in which the 

 temperature of the water had fallen from 65*1, as shown 

 by the last previous observation, to 60*2. The mixing of 

 the waters of the Tagus with those of the ocean is clearly 

 shown by the fall in temperature, the temperature ofi" the 

 mouth being on the run out 62-525. 



This brings us directly to a difficulty raised by the 



