at station 08-12, 13 miles southwest of Dunkirk, and 

 that this station had the most intense thermocline of 

 the season. Here the temperature at 1 8 meters was 

 18. 2° and at 18. 5 meters only 11. 6° --a difference 

 of 6. 6° in 0. 5 meter. These readings were taken on 

 a calm, smooth day, and their conectness estab- 

 lished beyond doubt by readings with different ther- 

 mometers. (Indeed all marked thermoclines ob- 

 served during the season were checked by observa- 

 tions with a different set of thermometers. ) The 

 depth of the thermocline at station 08-61 was 28 

 meters. The relation between stations 12 and 61 in 

 this respect was almost identical with the conditions 

 on cruise 6. 



Temperatures in 1928 



The 1928 program did not extend west of Long 

 Point and therefore did not include the western limit 

 of the Deep Hole, or any of the Great Plain area. 

 There were, however, three regular cruises when 

 stations 1 to 24 were occupied for physical data. 

 During this period the mean surface temperature was 

 1. 3° C. above that of the same stations on the cor- 

 responding cruises of 1929, but the mean bottom 

 temperature was only 0.4° above the mean in 1929. 

 The average of the mean of temperatures at the sur- 

 face, at the 10 -meter level, and at the bottom was 

 1. 0° above the mean in 1929. The area under con- 

 sideration appears to have received a little more 

 heat up to September 1 in 1928 than in 1929. The 

 data are not sufficient, however, to permit a def- 

 inite conclusion. 



On August 16, 1928, during calm weather, sur- 

 face temperatures were fractionally over 24° C. in 

 Long Point Bay, whereas during the corresponding 

 time in 1929 the surface water of this area was be- 

 tween 18° and 19°. The surface temperature of a 

 given area is subject to great fluctuation due to vary- 

 ing weather conditions and that general thermal 

 values are obtained only by averaging many stations, 

 or by the means of frequent observations at the same 

 station over a considerable period. The high surface 

 temperatures in Long Point Bay on the above date 

 were caused by the calm, clear weather of that day 

 and by the light easterly winds of the four preceding 

 days. The warm surface water was slowly carried 

 to leeward while the cold bottom waters moved to 

 windward. The low -velocity winds caused no great 

 vertical mixing. The presence at this time of 5° 

 bottom water 10 miles east of Dunkirk is evidence 

 that the movement took place. 



As the prevailing winds are from the west, one 

 would expect to find warmer surface water along 

 the American shore and in the Buffalo region. This 

 expectation was met on four of the seven regular 

 cruises during the two summers. On two of the cruises, 

 the horizontal gradient was slight, and on only one 

 the warmer water was in Long Point Bay and along 

 the north shore. 



The remarkable thermocline at station 17 was 

 first observed on July 31, 1928 (table 7 gives the 

 thermocline data on 11 days during the two seasons). 

 The wide range in depth and the fact that on half the 

 observations no thermocUne existed show that the 

 cold waters of the Deep Hole were in motion vertically 

 as weU as horizontally. It is significant that on the 

 three cruises when thermocUnes were observed at 

 station 12 (table 6 -A) there were none at station 17 

 (table 6-D). These stations are on opposite sides of 

 the Deep Hole, and lie along a northwest -southeast 

 line. 



The shifting thermocline at station 17, and the 

 great horizontal movements of the cold water mass 

 observed in 1928, were witnessed again in 1929, and 

 undoubtedly represent normal summer conditions. 



Table 7. --Thermocline data at station 17 

 (Long Point) 



Summary of Temperature Observation 



The mean temperatures for the whole lake for the 

 period of the 1929 survey (May 15 to September 19) 



52 



