(f) It can be made in the city or town where it is to be installed, thus giving the locality the advantage of having some of the money paid for labor in its manufacture spent in the place where the sewers are being put in, where it is raised as a tax, etc.; also saving freight charges, etc.

(g) It can be made under the most careful local supervision and inspection, of selected material, by the engineer who is responsible for the success of the work. Vitrified pipe can seldom be made in this way.

DISADVANTAGES OF CONCRETE PIPE.

(a) If not carefully made and of the best of materials, it is subject to failure by disintegration, etc.

(b) It will not stand strong chemical action, and therefore the smaller sizes should not be used where they are likely to be exposed to trade wastes containing strong acids. In the larger sizes the quant.i.ty of flow mixes so quickly with the trade wastes that this danger is minimized, and it is very seldom that even the smaller sizes become affected; but vitrified pipe may be used in places where chemical action is antic.i.p.ated.

(c) If not properly made, it will be attacked by steam and hot vapor, and by animal fats in the sewage; but, if properly made, it is not affected by these.

(d) Unless reinforced or made very thick, it will not stand as great a crushing load as the best vitrified stoneware pipe; but, as sewers are not intended to be used under very heavy pressure, this is not so very important. It is amply strong to withstand any internal pressure or any external crushing load to which it probably will be submitted.

(e) Under a considerable head of ground-water, it may permit water to infiltrate through its walls for a considerable time after it is laid, thereby temporarily increasing the flow, which, if the sewage is to be pumped, will increase the cost of pumping.

This difficulty can be met by using a carefully selected mix of materials in making the pipe, and by making the joints carefully. Infiltration through concrete diminishes rapidly after the sewer is in use; it occurs in vitrified pipe, also, to some extent.

The house connection drain adopted in Monterrey, with the disconnecting trap, is very much like one which the writer has seen introduced with very bad result. These are being removed as rapidly as possible by one of his clients, a sewerage company, in the Southern States. It has been a fruitful cause of stoppages and bad smells; the ordinary method now in general use is much better. In the design shown, it would seem that there may even be some danger that the ventilation of the sewer by the stand-pipes in the streets may force the traps.

One is rather surprised to learn that the main outfall sewer is designed with a capacity of 90,000,000 liters per day, the present sewage being estimated as not more than 18,000,000 liters, and the far future being thought to require only 40,000,000 liters. Why this excessive size?

Possibly the surplus water which it is to carry is to be discharged into the sewers from the water supply system direct, and is intended for irrigating the land at the disposal area, when the sewage is insufficient for this purpose. The author states that all surface water is strictly excluded.

The method of sewage disposal gives rise to several questions. It is proposed to use an extensive area for growing crops, which are to be irrigated with sewage. The paper states that the underlying strata at Monterrey contain numerous caverns, and the first question is: What will be the effect on the water supply of other towns lower down the valley?

The writer recollects a serious outbreak of typhoid fever in Bluefield, W.

Va., caused by the pollution of the water in similar strata finding its way through unknown underground caverns and channels to the city's water supply.

The next question is: What crops will be grown? It is a well-known fact that vegetables grown by the use of sewage as a fertilizer, are unsafe in a raw state for human consumption. This is well-known to European travelers in China and j.a.pan, where the use of fecal matter as fertilizer renders the various water supplies (where not filtered and disinfected) and all green vegetables, unsafe, on account of typhoid germs. Moreover, crops not intended for human consumption, which are grown on lands irrigated by sewage bearing typhoid germs, etc., are unsafe for men to handle; even to store them may cause a dissemination of disease. It is evident, therefore, that the whole sewage flow should be in some manner disinfected at least, if not filtered, before it is used.

The method of sewage disposal and the use of merely settled septic sewage for irrigation seem to be open to objection. The disposal plant is not sufficiently effective to meet the present requirements of sanitary science; and the sludge-pit will be certain to breed a pest of flies, if it is not also an intolerable nuisance on account of foul smells.

Monterrey would seem to be a proper place for the introduction of the Imhoff tank, with percolating filters, and a final settling tank, the effluent being disinfected, before entering the latter tank. The flow might then be used safely for irrigation purposes for crops not to be eaten uncooked by man. The writer does not see how the method provided can possibly fulfill the object stated, to distribute on the land an effluent which will be "innocuous and clear," or how any consequential degree of purification can be obtained in the tanks provided.

While there are described in this paper many things to find fault with, there are also many things to commend. The water supply system, with its reservoirs, etc., seems to be admirable; and the methods of construction by which the expense for forms was reduced is very interesting. The parking and ornamentation of the grounds over the reservoir roofs cannot fail to benefit the people and popularize the work.

RUDOLF MEYER, M. AM. SOC. C. E. (by letter).--The writer, as Engineer for the Government (guaranteeing the concessionaires a gross return of 10% per annum on the capital invested), and as inspector of the various works has had exceptional opportunities to become acquainted, not only with their construction, but also with events leading up to the granting of the final concession under which they were built and will be extended. In order to judge of the extent to which the different engineers, in their turn contributed toward the design of these works, the writer has thought it desirable to submit a complete statement of all matters relating to the inception, investigations, surveys, tests, etc., previous to the adoption of the present plans.

Data regarding former investigations, plans, and concessions which have since lapsed, have been obtained from the Government archives. These refer to periods prior to Mr. Conway's engagement, and anterior to the retaining of Mr. Schuyler by the concessionaires, and Mr. Binckley's connection with the scheme, and they are presented here as complementary to the information in the paper.

Samuel M. Gray, M. Am. Soc. C. E., acting in the interest of some American capitalists (who had been induced by Col. J. A. Robertson, of Monterrey, to look into the merits of a concession acquired by him, for building these works), being guided by the Government's proposition to supply the city with water by damming the flood-waters of the Santa Catarina River in the narrow gorge through which the stream emerges from the Sierras, some eight miles from the city, had several soundings made and reservoir sites surveyed in the first two box canons up the river, and prepared and presented to the Government several alternative projects, besides the one mentioned by Mr. Schuyler. Several different dam sites were designated by Mr. Gray, whose investigations extended over some two years, and were finally abandoned after he had designed the general outlay for a complete network of water mains and sewers for the city, on account of the unwillingness of the Government at that time, about 1897, to grant any guaranties as to bonds or income to the concessionaire or his a.s.signs. Mr.

Gray did not favor the general scheme of storing flood-waters as a water supply, but strongly recommended to the attention of the Government the greater advantages of deriving the supply from subterranean flow in the river, by an infiltration gallery driven into the water-bearing gravels in the Santa Catarina Canon (only a short distance above the place where Mr.

Binckley afterward established his bore-holes across the river). He proposed to take advantage of the steep slope of the river at a turn in the canon, and from the lower end drive a tunnel through a projecting rock spur, which tunnel, though starting well above the ordinary reach of floods, would terminate in water-bearing gravel, at a sufficient depth below the surface of the river-bed to intercept part of the underflow. Mr.

Gray, through investigations made under his direction, by Nathaniel Turner, M. Am. Soc. C. E., had ascertained that there was an abundant subterranean flow, and work had actually been started on the proposed tunnel.

The results of Mr. Gray's investigations were put at the disposal of Messrs. Mackenzie, Mann & Co. by Mr. Robertson, at whose offices Mr.

Binckley prepared the first plans submitted by him for the approval of the Government.

After Mr. Gray's investigations, Messrs. Mackin and Dillon (F. H. Dillon, a.s.soc. M. Am. Soc. C. E.), under contract with the Government, prepared the following plans: For a dam in the Santa Catarina Canon; for a pipe line, similar to the one proposed by Mr. Gray, to a reservoir and settling basin on the left bank of the river (a short distance above where the provisional pumping station was established afterward by Mr. Binckley), but on the flat above the bluff skirting the river, practically at the same elevation as the present high-pressure reservoirs; for a complete network of water mains and sewers in the city, indicating the approximate direction in which the sewage would be disposed of, either by turning it into the river or by spreading it over suitable lands, the location of which was to be determined later; and also a complete set of specifications.

On these data bids were invited by publication, and inquiries were received from several parties. Finally, Messrs. Stocker and Walker, of Scranton, Pa., entered into negotiations with the Government, and the present concession was agreed upon and granted.

Messrs. Stocker and Walker engaged the late E. Sherman Gould, M. Am. Soc.

C. E., to prepare a plan for a storage dam in the Santa Catarina Canon, and submitted plans for water distribution and sewers in the city, slightly modifying the original plans of Messrs. Mackin and Dillon.

In the fall of 1905, the concession was acquired by Messrs. Mackenzie, Mann & Co., of Toronto, Canada, together with all plans, etc., presented by the original concessionaires. The new concessionaires stated that they would examine the whole situation again, for the purpose of presenting modified plans for works.

Mr. Schuyler, in the interest of the new owners, had paid one flying visit to Monterrey when Mr. Gray's projects were brought to his notice, and the writer had an opportunity to show him the tunnel which had been started.

Mr. Schuyler left for Brazil and did not return until February, 1906, when he was accompanied by the Chief Engineer appointed by the concessionaires.

Messrs. Schuyler and Binckley then prepared plans for the water distribution and sewer systems in the city and for a provisional water supply to be pumped at San Geronimo, some two miles up the river. The new plans for the city work followed closely the general disposition by Mr.

Gray, the princ.i.p.al difference being that the main reservoirs for the permanent water supply were located to the south instead of to the west.

This change was due to the results of an investigation, made during Mr.

Schuyler's absence in Brazil, by Mr. F. S. Hyde, late Hydraulic Engineer of the Necaxa Water Power plant, who, accompanied by the writer, visited the whole water-shed of the Santa Catarina River in October, 1905, in search of suitable dam sites and prospects of power development. Mr. Hyde extended his studies to the Santiago Canon, southeast of the city, recommending finally that the water be brought from that canon, and that wells be dug in different points of the Santa Catarina River between San Geronimo and the entrance of the canon, and tested by pumping, for the purpose of establishing levels and ascertaining the available amount of underflow, with a view of determining the location for an infiltration gallery high enough up the river to permit of a gravity delivery and under good pressure in the city.

In view of Mr. Hyde's report, and as the result of a visit to the Santiago Canon, Mr. Schuyler decided to locate the reservoirs south of the town, intending to bring in water from the southeast, from springs in the Santiago Canon, and also by infiltration from Santa Catarina, his and Mr.

Binckley's scheme of water supply being for the same pressure throughout the city.

To supply water during construction, and partly meet the demands of the city, Mr. Binckley, on his arrival, decided to establish a provisional pumping station at the well in the river nearest to town, started by direction of Mr. Hyde at San Geronimo. This well was situated within the bed of inundation of high floods, on a low bank, at the foot of a conglomerate bluff some 20 ft. high, limiting a flat which was above the reach of any flood. It was on the same side of the river as the city, and there was plenty of good ground on the flat above for the establishment of a reservoir.

A slightly shorter pipe line was secured by crossing the river, building the reservoir (a substantial concrete-lined and vaulted-over structure) on the opposite bank, laying out the pipe line to follow that bank nearly to the city, and finally crossing back again; but the result has been that since the flood of August, 1909, in which the river crossings were destroyed, the reservoir remains isolated on the other side of the river from town, though intended to form part of the permanent works and act as a compensating reservoir for equalizing the pressure of the high-pressure system. Fortunately, the pumping station, the larger pumps, and the boilers, had been moved up the bank (after a rapid rise in the river on August 10th, 1909) to the new wells established by Mr. Conway on the line of the proposed prolongation of the infiltration gallery. The reservoir, however, is left to stand alone on the other side of the river, and its usefulness will not be restored until a new line is laid across the river, re-establishing its connection with the new pump line and the new and permanent pipe line to be laid along the north bank from the pumping station to the city. This will free Monterrey from the constant menace of a water famine. At present its two main water supplies may be cut off by unexpected floods like those of 1909 and 1910, as both supplies are carried across the river, and though only the cast-iron pipe connecting with the water supply from Estanzuela was carried away by the flood, the concrete conduit of the San Geronimo low-pressure supply was seriously threatened. Such risks are too great to be carried for any length of time; besides, a succession of dry years would cause such a reduction in the Estanzuela supply as to require an additional reserve in the way of pumping stations drawing on the under-flow of the river, such as already exists in San Geronimo.

Afterward, Messrs. Schuyler and Binckley submitted preliminary plans and profiles for the projected concrete gravity conduit from Estanzuela to the reservoir south of the city, and Mr. Binckley submitted excavation plans for two reservoirs, only one of which was built, and from designs by Mr.

Conway.

Stephen E. Kieffer, M. Am. Soc. C. E., was intrusted by Mr. Binckley with the revision of the plans of the water distribution and sewers. The southern half was approved by the Government and executed according to his plans; the northern part was afterward revised by Mr. Conway and has been partly built.

The final maturing of the project of an infiltration gallery in San Geronimo as a low-pressure gravity supply, the division of the city into high- and low-pressure districts corresponding to the two supplies, with one reservoir, instead of two to the south of the city, and the other to the west at the Obispado, the entire details of both these gravity schemes, and of the whole sewage disposal scheme, as well as the modification introduced into the city work for the northern half, are unquestionably due to Mr. Conway, independently of the general views which may have been held on those points by other engineers.

In March, 1910, Mr. Conway left Monterrey, all the princ.i.p.al works being finished. Since that time Vicente Saucedo, a.s.soc. M. Am. Soc. C. E., has put in many additional water mains and sewers in the northern part of the city, and is finishing the _force majeure_ work caused by the destruction wrought in the districts along the river banks by the extraordinary floods.

The writer, having had an opportunity to watch the earnest efforts of the several engineers connected with these works, in the course of their design and construction, resulting without doubt in being the first of their kind built in Mexico, has been induced to contribute this discussion in order to bring out clearly the share of each.

Mr. Pitkethly's apprehensions as to the adequacy of the system of ventilation adopted have not been realized, in part perhaps because the houses, though generally of only one story, have such high ceilings that the tops of their vent pipes are generally higher than the ventilating columns at the heads of the branch sewers.

GEORGE ROBERT GRAHAM CONWAY, M. AM. SOC. C. E. (by letter).--The writer regrets that some features of the works described in this paper have failed to call forth the many useful criticisms which he expected, and his remarks, therefore, are limited to the few points which have been raised.

He is particularly indebted to Messrs. Schuyler, Meyer, and Saucedo for adding supplementary information of value to the paper, but regrets that he cannot support Mr. Binckley in his claim that "the entire general design of the system, as well as the extensive hydrological studies and final selection of the sources of water supply, was completed in 1906,"

etc. On May 1st, 1907, when the writer a.s.sumed responsibility as Chief Engineer, he was unfortunately confronted with the fact that very little data and only a few preliminary and incomplete plans were available. His first duty was to report upon the final sources of supply, and the recommendations made in his report (dated July 12th), received Mr. (now Sir William) Mackenzie's approval during the same month. The final plans, upon which the approval of the State Government was definitely obtained, were prepared by the writer during the summer of 1907, were submitted to the Governor of the State, Gen. Bernard Reyes, on October 19th, and received his approval on December 12th, 1907. No works, with a long preliminary history, such as those at Monterrey, can rightly be said to be due to any one individual; many engineers contributed to the final result, and the writer willingly acknowledges his indebtedness to the able men, who, for ten years prior to the construction of the works, investigated the particular problems which were met--problems which were not only of an engineering and physical nature, but racial and financial. The responsibility of constructing the works in their present form, and leaving them practically complete, did, however, fall on the writer's shoulders.

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 antic.i.p.ate 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 quant.i.ty was absolutely unknown. The writer's practice in other places has been to a.s.sume 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. CLx.x.xI, 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.

"(2) That the average velocity of the air escaping up these columns was 3.2 feet per second, representing the circulation of 3,600,000 cubic feet of air per diem, or sufficient to change the air in the sewers every 10 minutes.