240 Temperature of the River Nith. 



that of the air or land. The highest temperature recorded for 

 the river was 74 deg., on the 20th of July, and the lowest was 

 31.5 deg., say 32 deg. to avoid fractions, on the loth of 

 February, giving a range of 42 deg. for the year. In the air or 

 land observations, on the other hand, the highest temperature 

 was 90 deg., on the same day in July, and the lowest 6 deg., on 

 a day in February, which would give an annual range of 84 deg. 

 As the above values, however, were abnormal, I prefer to substi- 

 tute for them a fifteen years' average of the highest single day 

 temperatures, and a similar average of the lowest, which would 

 alter the range to 84 deg. for the highest and 14 deg. for the 

 lowest, and show an annual range of 70 deg. for the air, as com- 

 pared with 42 deg. for the river. The mean maximum for the 

 year being 49.9 deg., and the mean minimum 46.3 deg., the 

 mean daily range would only be 3.6 deg. For the air the mean 

 maximum was 54.4 deg., the mean minimum 40.5 deg., hence 

 the mean daily range would be 14.4 deg. But now, when we 

 compare the mean annual temperatures of air and water, we find 

 a remarkable agreement. They almost exactly coincide. The 

 one (air) is 47.7 deg. ; the other (water) is 47.8 deg. The differ- 

 ence amounts to only one-tenth. I may mention that authorities 

 on the subject lay it down as a rule that the temperatures should 

 coincide. Thus Dr Hugh Robert Mill, in his book, entitled 

 "The Realm of Nature," says: — "The temperature of a river 

 in the temperate zone follows that of the land over which it 

 flows, and it is thus subject to considerable variations between 

 day and night." Hence the use of self -registering minimum as 

 well as maximum thermometers to record the temperature is 

 fitted to secure greater accuracy ; and when the yearly mean, 

 founded on such observations, is found to correspond with that 

 of the air for the same period, it is only what might be expected, 

 and may be regarded, indeed, as an evidence of the accuracy of 

 the observations. This coincidence of annual means does not, 

 however, necessarily imply a like coincidence in the monthly 

 means. It will be seen from the table that the months in which 

 they approximate most closely are the autumn and winter months, 

 and those in which they diverge most are in spring partly, but 

 still more in summer. In August, September, October, 

 November, December, February, and April there is only a 

 difference of a fraction of a degree, but in January, March, May, 



