------------+------------------+-------------------------------------- Normal Mean | | January | Place. | Kind of Filters and Results. Temperature.| | Degrees F. | | ------------+------------------+-------------------------------------- 37-40° |All English cities|Open filters only are used, and no | | great difficulty with ice is | | experienced. 33-35° |Cities in Holland |All filters are open, and there is | | little serious trouble with ice; | | but at Amsterdam and Rotterdam | | the bacteria in effluents are said | | to be higher in winter than at | | other times. 32° | Bremen |Open filters. 31° | Altona |Much difficulty with ice in open | | filters (see Appendices II and VII). 31° | Brunswick |Open filters. 31° | Hamburg |Open filters. 31° | Lübeck |Open filters. 31° | Berlin |Open filters were formerly used, but | | owing to decreased efficiency in | | cold weather they have been | | abandoned for covered ones. 31° | Magdeburg |Covered filters, but a recent addition | | is not covered. 30° |Frankfort on Oder |Open filters. 30° | Stuttgart |Part of the filters are covered. 30° | Stettin |Part of the filters are covered. 29° | Zürich |Covered filters were much the most | | satisfactory, and the open ones were | | covered in 1894. The raw water has a | | temperature of 35°. 29° | Liegnitz |Open filters. 29° | Breslau |Open filters have been used, but | | recent additions are covered. 29° | Budapest |Covered filters only. 29° | Posen |Covered filters only. 26° | Königsberg |The original filters were open, but it | | was found necessary to cover them. 24° | Warsaw |Covered filters only. 16° | St. Petersburg |Covered filters only. ------------+------------------+-------------------------------------- In case the raw water was drawn from a lake at a depth where its minimum temperature was above 32°, which is the temperature which must ordinarily be expected in surface-waters in winter, open filters might be successfully used in slightly colder places.