Lake Maxinkuckee, Physical and Biological Survey 61 



The pressure for August, 1900, was remarkably uniform, the 

 graph for this month standing in marked contrast with all the 

 other months considered. There were no rapid or violent changes 

 whatever. The average for the whole month is just a trifle below 

 the average for the whole period, and the pressure for the middle 

 third of the month is somewhat lower than that for either end. 

 While there are no violent changes in pressure whatever there is 

 a remarkably regular daily pulse of variations. With only two 

 exceptions the morning reading is the highest; from this time 

 there is a fall of about .16 inch (varying from about .09 to ,2 

 inch) until noon, after which there is usually very slight change 

 toward evening, after which it rises back to nearly its original 

 morning reading. We have then for this month a rather constant 

 daily mode. This mode is of quite frequent occurrence through- 

 out the whole year, and there are all degrees of approximation 

 to it and divergence from it ; during the autumn, winter and early 

 spring months its appearance is somewhat less frequent. Rather 

 strictly interpreted, this mode occurs with the following coefficient 

 of frequency for different months: August, 21; September, 4; 

 October, 13; November, 5; December, 2; January, 2; February, 

 5; March, 4; April, 9; and May, 4. 



The average height of the barometer is determined by the alti- 

 tude of the place, and the great barometric changes are due to 

 the passage of general storm centers. The small daily changes, 

 however, are quite probably due to local conditions, and even the 

 large changes are due to conditions which are represented on a 

 small scale in the area immediately about the lake. Among these 

 causes are difference in nature of the surface of areas of land and 

 water and the presence in the atmosphere of clouds, vapor, dust, 

 etc. 



Generally speaking, the air mass over a water area is com- 

 paratively stable in relation to daily changes of temperature, while 

 that over land areas is subject to much larger changes. We have 

 differences somewhat analagous to differences of potential between 

 different elements in a galvanic cell. The air above the land sur- 

 faces becomes superheated during the day and during the after- 

 noon represents areas of low pressure, while the air mass above 

 them becomes cooled during the night and becomes areas of high 

 pressure during the latter part of the night. 



Again taking up land surfaces in detail, these differ consider- 

 ably among themselves. Large areas of sand have a greater daily 

 amplitude of temperature than areas of loam or muck, and these 

 more than clay, so that, continuing the analogy of the galvanic 



