--------+----------+-------------+------------------------------------ | | | Persons who are known to have been Years. | Total | Deaths | exposed to infection. | Number | per +--------------+--------------------- | of | 10,000 | | While living out | Deaths. | of | By drinking | of town just before | | Population. | Canal Water. | falling sick in | | | | Lawrence. --------+----------+-------------+--------------+--------------------- 1888 | 48 | 11.36 | | 1889 | 55 | 12.66 | | 1890 | 60 | 13.44 | | 1891 | 55 | 11.94 | | 1892 | 50 | 10.52 | | 1893 | 39 | 7.96 | | 1894 | 24 | 4.75 | 12 | 1895 | 16 | 3.07 | 9 | 2 1896 | 10 | 1.86 | 2 | 4 1897 | 9 | 1.62 | | 1898 | 8 | 1.39 | 1 | --------+----------+-------------+--------------+--------------------- Filter put in operation September, 1893. Average rate before the introduction of filtered water (1888-92) 11.31 Average rate afterward (1894-98) 2.54 These results show a striking reduction in the deaths from typhoid fever with the introduction of filtered water, which has been most gratifying in every way. The more recent history of the underdrains of the Lawrence filter is particularly instructive. Owing to the absence of a water-tight bottom to the filter, and its low position, a certain amount of water constantly entered the filter from the ground below. This water contained iron in solution as ferrous carbonate. When this water came in contact with the filtered water in the gravel and underdrains, the iron was oxidized by the dissolved oxygen carried in the filtered water and precipitated. This was accompanied by a growth of crenothrix in the gravel and underdrains, which gradually reduced their carrying capacity. This reduction in carrying capacity first became apparent in cold weather when the yield from the filter was less free than formerly. There was difficulty in maintaining the supply during the winter of 1896-7 and more difficulty in the following winter. [Illustration: FIG. 16.—TYPHOID FEVER IN LAWRENCE, 1888 TO 1898.]