Hilgkndorf. — Artesian Wells in the Ghristchurch Area. 151 



The graph of the well is shown by the full line, and its variations in 

 centimetres ; that of the barometer by a dotted line, and its variations in 

 inches. It will be observed that the scale for the well increases upwards, 

 and that for the barometer downwards ; further, the space between 29-82 in. 

 and 30-30 in. on the barometer scale (virtually Jin.) is the same as the space 

 between 86 and 82 on the well scale — that is, 4 centimetres (virtually 2 in.). 

 This means that the close agreement of the two graphs seen in fig. 6 has 

 been obtained by turning the barometer-readings upside down and multi- 

 plying by four, as above stated. Figure 6 is a portion of a graph that 

 was constructed for the whole year for the purpose of establishing the 

 agreement between the fluctuations of the well and the barometer. 



The section 22nd March to 8th April was chosen for illustration because 

 it is fairly typical of the whole graph, and because there is no complication 

 due to rainfall. There were showers on the 29th, 30th, and 31st March, 

 and on the 1st, 3rd, 4th, and 6th April, but the three heaviest of these 

 were only 0-07 in., 0-08 in., and 0-09 in., and the remaining were 0-01 in. 

 each. These numerous rains, the greatest of which did not reach 0'1 in., 

 cannot be suspected of influencing the graph of the well, since in any case 

 the well does not always rise after the rainfalls shown. 



That a low barometer is accompanied by a rise in the static level of 

 shallow wells has been excellently demonstrated by F. H. King, and for 

 artesians is recorded in the following sentence occurring in an article by 

 Professor J. W. Gregory in the " Journal of the Eoyal Geographical Society " 

 for August, 1911, p. 171 : " The Hon. E. W. Lamb kindly tells me that an 

 increased flow has been observed in some of the wells of New South Wales 

 at times of low barometric pressure. The increased flow from springs when 

 the barometer is low is a well-known phenomenon which has been esta- 

 blished, for example, by the work of Mr. Baldwin Latham near Croydon. 

 The increase is no doubt due to gas-pressure, the gases dissolved in the 

 water expanding when the atmospheric pressure is reduced. Mr. Latham's 

 evidence therefore shows that gas-pressure acts even on wells of which the 

 flow is mainly determined by ordinary water-pressure." 



I have examined this theory of the rise of wells under diminished 

 atmospheric pressure. It appears to assume that water is compressible, 

 or, at least, that the gases within the water are compressible. This, of 

 course, is not so. If the pressure is diminished the gases will remain 

 dissolved if the water is not already saturated with them, and if the 

 water is saturated the gases will come out of solution and form bubbles. 

 The water in the College well is saturated, containing 26-50 c.c. of gases 

 per litre at N.T.P., made up as follows : Carbon-dioxide, 1-07 c.c. per 

 litre ; oxygen, 4-29 c.c. per litre ; nitrogen, 21-14 c.c. per litre. 



It must be further remembered that the volume of a gas absorbed 

 by water is independent of the pressure, since, although doubling the 

 pressure doubles the mass of the gas absorbed, the same doubling of 

 the pressure halves its volume. If, then, the pressure were suddenly 

 diminished the volume of gases liberated would be proportional to the 

 diminution of pressure, and if the gases remained in suspension in the 

 water the volume of the water would be increased. 



Calculation shows that with a diminution of atmospheric pressure 

 from 30 in. to 28 in. of mercury — that is, from 15 lb. to 14 lb. per square 

 inch — the bubbles of gas liberated in this well 340 ft. deep would raise 

 its level by 1-8 in. By observation, the rise of the well under such a 

 barometric fall amounts to 8 in., and therefore the liberation of gases 



