302 



THE WATER-SUPPLY OF TOWNS IN THE TROPICS 



The air-lift 

 pump 



Doubts as tc 

 the quality of 

 the supply 



stratum was tho wator-boariag sandstone which was shown as coarse and containing gravel. 

 The earlier sections showed the rock to be seamed at different levels with clay marl. 



Tliere had apparently been a fault between Well No. 7 and the other wells, for, in its 

 case, below 23 metres, the strata dilTered in position and thickness. One need not, however, 

 go into details, nor is it necessary here to enter into the question of the quantity of water 

 yielded at first by these wells, as our space is so limited, but a word or two may be devoted 

 to the air-lift pump, the method of water-lifting eventually employed. The air-lift, which is 

 not strictly a pumping machine, was, I believe, originally an American invention which, under 

 certain conditions, renders excellent service. Its principle of action is the lessened specific 

 gravity of water in which air is contained in considerable quantit}', or through wliich it is 

 rising in bubbles. There is an air-pump or compressor, and from it an air-pipe passes to the 

 bottom of a water tube, into which tube air is discharged. The air rising, causes the surface 

 of the water to rise, since the atmospheric or other pressure at the base remains the same. 

 The output from deep wells can thus be increased although the total mechanical efficiency 

 of such a plant is said to be less than that of a good pumping installation. One mentions it 

 here as, naturally, its influence on the bacterial content of the water required consideration. 



As may be surmised, the very scanty information obtainable regarding these wells did 

 not place a bacteriologist in a very favourable position for coining to definite conclusions 

 regarding the results he might obtain on analysis. The responsibility was great, the 

 problem novel, and there was determined opposition to sanitary interference. One could 

 not tell if heavy rains, falling on a cracked and gaping surface soil, might not wash impurities 

 to such a depth that they could enter the well-water by the unguarded bore holes ; one could 

 not tell if, under the influence of powerful pumping, with great depression of the water 

 table, river water or water from pools in the dried river bed might not similarly gain 

 entrance. From the river bed to the top of the unprotected bore hole of Well No. 5 

 there was only 13 feet of sand in a vertical direction, and 90 feet of presumably porous 

 material in a horizontal direction to act as a "natural" filtering medium. Again, there 

 were the shallow native wells in the neighbourhood wliich certainlj' merited some 

 consideration. One felt doubtful about the whole scheme. There might be little evidence 

 of pollution and yet the water might, under certain conditions, be a source of danger. 

 One had been confidently assured that the water was the finest in the world and had replied 

 with the expressive Arabic word " Yimkin," which, being interpreted, signifieth "perhaps." 

 If, at first, the results were good, it might be difficult to prevent the water being 

 rushed upon the town although it was evident that the tests would have to be 

 prolonged and careful. The influence, if any, of the falling and rising Nile, upon the wells, 

 certainly called for study, as did the effect of severe and exhaustive pumping. 



It was, I confess, an anxious time, and one was scarcely prepared for the results 

 which were obtained. Before mentioning these, one must outline the methods of 

 examination employed. 



Water was collected from the wells in special sterilised glass-stoppered bottles. 

 The discharge was, as a rule, from a special wrought-iron pipe of small diameter, leading 

 from the large delivery-pipe directly connected with the well. The small pipe was 

 passed through the furnace every morning of the days on which samples were taken. 

 Its faucet was also flamed prior to collection of the samples. As a rule, three samples 

 were taken from each well and at least two of them were examined. The bottles, after 

 their necks and stoppers had been flamed, were filled, packed in ice and transported by 

 motor or launch to the laboratories, where the examination was commenced as soon as 

 possible, nearly always within half-an-hour of the time of taking the samples. 



