Messrs. Pitkethly and Hammond have criticized the basis of the calculations upon which the sewer system was laid down. In considering this problem, it is necessary to remember that, in designing this system, there was practically no information upon which to base the calculations; and the writer believed that the wisest course was to anticipate a liberal growth, and provide a large margin of safety. In designing a sewer system in older and well-established communities, the engineer is generally able to compile considerable information regarding the probable sewage flow for which it is necessary to provide. In Monterrey this quantity was absolutely unknown. The writer's practice in other places has been to assume that about 8% of the total daily discharge of sewage will flow off in one hour; and, from many curves which he has plotted regarding sewage flow in British towns, this rate appears to him to be approximately correct. In Monterrey, however, the old Mexican traditions are rapidly changing, and the city is now becoming one of the most Americanized in Mexico; the old one-story houses will give way in time to buildings of several stories--a change, already noticeable, which has occurred during the past few years, particularly in the business portion of the city. Taking these facts into consideration, it is believed that it would be, not only bad engineering, but bad business, for a company whose concession lasts 99 years, to provide sewers as small as 6 in., as Mr. Hammond would recommend, and then be called upon, under the terms of the concession, to relay larger sewers at a future date, thus incurring further capital expenditure upon which no Government guaranty would apply, and no further revenue be obtained. In matters of this kind, not only the engineering, but the commercial, aspect of the question must be kept in view, and this point, the writer must frankly admit, he has always seriously considered. The writer's experience with reference to the method of ventilating sewers by tall columns extends over many years, and he still maintains that no other system gives such satisfactory results. In this view he finds considerable support in a recent paper on "Salisbury Drainage," by Mr. W. J. E. Binnie,[11] written since the system at Monterrey was installed, in which the result of a series of experiments carried on during 1906-07 are given. At Salisbury, England, 68 ventilators, 6 in. in internal diameter, 30 ft. high, were connected to the main sewer by 6-in. stoneware pipes. They were placed about 540 ft. apart, and, from careful anemometer readings, the following conclusions were reached: [11] _Minutes of Proceedings_, Inst. C. E., Vol. CLXXXI, p. 317. "(1) That four ventilators all lying in the lower portion of the town acted sometimes as air-inlets and sometimes as air-outlets, and that the other sixty-four acted as air-outlets.