320 
materials attending low water. This is the month of greatest 
production in 1897, and also the one of lowest unutilized ni- 
trates, the latter not exceeding 1 part per million during the 
month. It isin the rising chlorine (Pl. XLIV.) that we have 
a suggestion of the degree to which sewage has made contribu- 
tion to the stream. The unutilized nitrates do not of course 
afford a measure of its quantity. 
The October pulse has a duration of 42 days,—from Sept. 21 
to Noy. 2,—with a maximum amplitude of 12.92 em.’ per m.* on 
the 5th. Its mean also falls on the 5th, 26 (25) days after that 
of the preceding pulse. This extended volumetric pulse is found 
to include within its limits two of the pulses of chlorophyll- 
bearing organisms (cf. Pl. LII.),one culminating Sept. 29 and 
the other Oct. 19. The month was one of continued hydro- 
graphic stability. The total movement in levels was only .6 ft., 
due mainly to the check in evaporation resulting from decline 
in temperatures. The temperatures in this month average 
about 64.5°, which is 6° to 8° higher than the average in other 
years of our records, The difference between the extremes is 
only 17° as compared with 23° in 1896 and 27° in 1898. The 
autumnal decline in this year has come later and progressed 
less rapidly, at least till the last ten days of the month (Pl. XI.), 
than is usuallythecase. Thecurve of the October, as well as 
that of the September, pulse is delimited on either side by 
declines in temperature. 
The chemical conditions in this month (Pl. XLIV.) are less 
stable than in September. The nitrates and nitrites move in- 
versely with the plankton,and both chlorine and free ammonia 
ascend rapidly to unusual heights, suggesting the presence of 
sewage in which decay had not yet progressed as far as was 
wont during warmer weather. This is doubtless the result of 
the Peoria sewage pulse, which as winter approaches extends 
down stream toward Havana, and under the stable low-water 
conditions of 1897 appears in exaggerated form. 
In view of the stable conditions, excessive fertilization by 
sewage, and abnormally high temperatures, it is not surprising 
