298 Birge — The Crustacea of the Plankton. 



These temperature diagrams, which give the weekly averages 

 of temperature, do not show the actual condition of tempera- 

 ture, and especially the temperature of the thermocline, on any 

 single date. The thermocline oscillates up and down under or- 

 dinary conditions of weather through a meter or more; and the 

 effect of averaging the observations of a week is to increase the 

 apparent thickness of the thermocline and thus to diminish the 

 rapidity of descent of temperature in it. Without any consid- 

 erable change either of wind or temperature the thermocline may 

 oscillate through 2 or even more meters. The action of severe 

 wind is much more apparent. Fig. 5 shows temperature dia- 

 grams for August 2, 24, 26, 27, and 28, 1898. It will be seen 

 that the diagrams for the 2nd and 24th of the month were 

 closely similar, although the surface water had cooled a degree 

 or more and the thermocline had descended about 1 meter. On 

 the 24th there was a decided fall in temperature of the air ac- 

 companied by violent winds from the northwest. The surface 

 water fell more than one degree in two days, while the thermo- 

 cline was temporarily depressed at the observing station more 

 than 4 meters. It lay on the 24th between 10 and 11 meters; 

 on the 26th between 14.5 and 16 meters. The temperature at 

 the bottom, 18 meters, was raised about 0.4°, at 14 meters 5.6°, 

 at 12 meters 4.3°, at 10 meters there was a loss of about 0.6°. 

 On the 27th, the wind having fallen to a calm, the thermocline 

 had risen nearly 3 meters, while on the 28th, with a gentle 

 south wind, it had risen still further, and the temperature curve 

 had greatly changed in form. During these three days the 

 temperature to a depth of 8 meters had varied very little — too 

 little to show in the diagram. This example of changes which 

 are going on all the time, shows the following facts: 1. The 

 isotherms of diagrams 3 and 4 represent only the average posi- 

 tion of the thermocline. 2. The decline of temperature in the 

 thermocline is ordinarily much more rapid at any given date 

 than is indicated by the average of the week. In other words, 

 the thermocline is not nearly as thick as the week's average 

 would indicate. 3. The greatest daily variation in temperature 

 during summer is found at the thermocline, where a range of 5 

 or more degrees may be registered in a day. These variations 



