52 MARSHALL O. LEIGHTON 



area may be subject to occasional malicious pollutions by visitors, 

 etc., then the sanitary analysis does not offer any helpful solution. 

 A single infectious intestinal discharge deposited directly in a reser- 

 voir might readily cause a typhoid epidemic, but the organic matter 

 would not, except under very fortunate circumstances, be detected 

 by the nitrogen determinations; and it is well to reflect that, under 

 the usual conditions, by the time the disease had made itself manifest 

 and attention directed to that reservoir, the infection would have passed 

 out of the reservoir. 



If the drainage area above described does contain population, 

 then the danger is always impending, and we may rest upon assump- 

 tions, nearly, if not quite, as positive as those quoted above for river 

 waters. It is quite significant in this connection to note that the 

 Commission on Additional Water Supply of the City of New York 

 made provision for filtration, although the upland areas proposed 

 as new sources are sparsely settled. More recently we have read 

 the opinions of our foremost authorities that the present Croton 

 supply should be filtered. It is doubtful if any of those authorities 

 would contend that the sole object of such filtration is to remove 

 turbidity, color, and odor. It is therefore held that the sanitary 

 analysis of upland conserved supplies is needless, because we can 

 determine the danger by inspection far more readily and surely. 



With reference to ground waters : We have interesting accounts 

 of cases in which it is asserted that the condition of pollution was 

 not detected by biological examination, but was revealed by sanitary 

 analysis. If close consideration be given to the descriptions of 

 premises that appear in these accounts, it will be seen that in every 

 case (so far as the writer is informed) a careful man would have 

 been justified in condemning those waters upon superficial examina- 

 tion, and without regard to analysis. Take for illustration the case 

 cited by Professor William P. Mason in a paper entitled, "Interpre- 

 tation of a Water Examination," which appeared in Science, Vol. 21, 

 No. 539, pp. 648-53. It appears from this that there was a certain 

 farmhouse in England, the residents of which had suffered severely 

 from diphtheria and typhoid fever. Examination showed that the 

 sewage discharged from the house entered into a dry-steyned cess- 

 pool, without overflow, about four yards from the well, both sunk 



