90 BULLETIN 852, U. S. DEPARTMENT OF AGRICULTURE. 



area on the sides of the sewer. The incrustation seemed slightly- 

 heavier, but the organic growth did not show any marked increase 

 over that at the time of the 1897 tests. 



In his discussion Mr. Horton accepts the value of n of 0.0117 for 

 1896. of 0.0127 for 1897, and of 0.0133 for 1900. (See Table 11, p. 69 

 for individual values of each observation.) He notes that the de- 

 posits of grease and of organic matter were greatest on the sides, 

 especially near the line of average flow; that the bottom was clean 

 scoured by sand and other heavy particles. 



In his discussion on these experiments, Rudolph Hering mentions 

 several cases where the deposit exceeded that noted by Horton. 

 Mr. G. C. Whipple brings out the point that the organisms which 

 grow in sewers are vegetable forms of a very low type, while those 

 found in water pipes belong to the animal kingdom — fresh-water 

 sponges, etc. 



The results of these tests would indicate that sewage undoubtedly 

 coats a cement surface to such an extent that an exceptionally smooth 

 interior surface would not long contribute to a high capacity, as 

 would be the case for most clear waters. By the same deposits, 

 on the other hand, a very rough interior might be so smoothed over 

 that the capacity will be increased, provided the sewage was suffi- 

 ciently diluted with surface or other ordinary waters, thus preventing 

 excessive deposits. 



Nos. 68, 69, and 70, Experiment H-2. — Basket-handle cement- 

 washed brick sewer, North Metropolitan sewerage system, Boston, 

 Mass. — Simultaneously with the tests described as Nos. 65, 66, and 

 67, Horton also made experiments upon the basket-handle section 

 adjoining the circular section. The shape was 6 feet by 6§ feet, but 

 otherwise similar in construction to the circular section. The general 

 description under the latter applies also to this section. 



The difference in the values of n for the two shapes would indicate 

 that a better surface is obtained in a circular form than upon plane 

 faces, or those requiring hand troweling. A review of tests for 

 carrying capacity shows this fact to be quite general. 



No. 71, Experiment M-4. — Cut-and-cover sections of the Esopus 

 division, Catskill Aqueduct, New York State. — Long reaches of the 

 Catskill Aqueduct were constructed as flow-line sections, as dis- 

 tinguished from the reaches under pressure. 1 These sections are of 

 the cut-and-cover type, a concrete horseshoe 17£ feet wide and 17 

 feet high. 



The invert, 16 inches thick, was laid first, in alternate panels each 

 15 feet long. After the forms had been filled — 



The work is screeded with a lG-foot length of 3-inch circular steel shafting, which 

 is rolled along the top of the forms, bearing on the web plates and the horizontal legs 

 of the angle irons riveted to them. Great care is taken to obtain a smooth finished 

 surface. 



For the walls and arch, steel forms in 5-foot units bolted into 15- 

 foot sections were used. From invert to arch any one section was 

 completed at one pouring. After the oiled forms were in position, 

 four men crawled between the forms, two on each side. As the 

 concrete, a wet mix in a ratio of about 1 to 3 to 5, was poured in 



1 Engin Itec. vol. 61 Jan. 8, 1910; vol. 02, Nov. 5, 1910; Water Works Handbook, Flinn, Weston and 

 Bogert, New York, 1910, p. 271. 



