114 Mr. H. M. Witt on the Variations in the 



1 imagine this circle has been long known for the case of the 

 point of projection being in the fields but it may have escaped 

 notice for the more general case. The equality between the fall 

 and the excursion for the angle of maximum range subsists, not 

 merely for a rectihnear or curved section, but for the ground 

 itself (whatever its form of surface) when the gun is supposed 

 to admit of being laid to any angle, as well as at any elevation. 



XV. On the Variations in the Chemical Composition of the 

 Thames Water ^ during the year between May 1855 and May 

 1856. By Henry M. Witt, F.C.S., Assistant Chemist to the 

 Oovemment School of Science Applied to Mining and the Arts^, 



SO numerous are the chemical analyses of the water of the 

 Thames which have been published during the last few 

 years, and by chemists of such eminence have they been made, 

 that it would appear altogether superfluous to make further ad- 

 ditions to their number ; but one moat important fact has scarcely 

 attracted sufficient attention, that any single analysis, however 

 carefully and accurately executed, represents only the composition 

 of that particular sample of water under examination, and not 

 the general or average composition ; in fact the quantities of the 

 several impurities of the water of the Thames diflfer, as might be 

 expected, most essentially at the various points in its course, and 

 at the several seasons of the year. 



To the variations in composition at different points of the 

 river's course attention has been drawn long since, especially in 

 the Report by Professors Graham, Miller, and Hofmann, to the 

 Secretary of State for the Home Department, made in 1851 f, 

 by whom it was recommended that the supply of London should 

 in future be taken from some point above the tidal influence, 

 further removed from the contamination of the London sewage. 



But great discrepancies have often occurred between the ana- 

 lyses of the water at any particular point made by different ex- 

 perimenters, due doubtless to the changes which take place at 

 different seasons of the year; and it became evident that a cor- 

 rect opinion of the composition of the water, at any special point 

 of the river's course, could only be formed by making a large 

 number of analyses extending over a considerable space of time, 

 and from them deducing the mean composition. 



This desideratum was partially supplied by Dr. Robert Dundas 

 Thomson, who, in the Quarterly Journal of the Chemical Society 

 for July 1855 J, published the results of a series of analyses made 



* Communicated by the Author. 



t London : Taylor, Walton, and Maberly, 



X Vol. viii. p. 97. 



