ON THE SEASONAL VAEIATIONS OF TEMPEKATURE. 471 



about tliree-quartei'S of a degree C. colder in the afternoon tLan in the 

 morning — the average of the month of June shows this afternoon cooling 

 to be as much as 1°'8 C. In the cold months the phenomena are reversed. 

 When the tide is high in the morning and low in the afternoon, the water 

 is about half a degree C. colder in the afternoon than in the morning — 

 in December as much as l°-0 C. When the tide is low in the morning 

 and high in the afternoon, the water is about three-quarters of a degree C. 

 warmer in the afternoon — in December as much as 1°"6 C. 



It thus appears that the tidal effect on temperature is stronger than 

 the solar. In summer, no matter how hot the day may be, the water at 

 the Abertay lightship cools steadily until the hour of high tide ; in winter, 

 no matter how cold the night may have been, the water warms steadily 

 until the hour of high tide. The explanation is simple and sufficient. 

 The temperature of the water of the Tay is always higher in summer and 

 lower in winter than that of the sea, and putting the case generally, the 

 Abertay light- vessel floats in Tay water at low tide, in North Sea water 

 at high tide. 



These observations, which are not further alluded to in this report, 

 serve to record a very large amount of thoroughly trustworthy observa- 

 tions. Especial attention is directed to Mr. Chapman's magnificent series 

 of mean monthly temperatures, which illustrate very clearly the varia- 

 tions in the seasonal swing of temperature in water in one of the most 

 extreme average climates of the British Islands, taking its low altitude 

 into account. It serves not only to record the variations in temperatures 

 of the Cherwell, but to suggest the range of deviations from normal 

 seasonal temperatures to be expected in any case. Thus we are warned 

 not to assume the mean of one or two years as being the real mean 

 temperature of any exposed body of water. 



B. Record of Occasional Observations. 



Many observations taken regularly at intervals of a week acquire some 

 value in tracing the seasonal variations if they have been carefully per- 

 formed and carried out at exactly the same hour on each occasion. 



RrvER Wajjdle. 



Observations ly Mr. F. C. Bayard, LL.M., F.E.Met.Soc, made under the 

 auspices of the Groydon Microscopical and Natural History Clul. 



These observations were made once a week at a number of points near 

 the sources of the river Wandle, which takes its rise as an outflow from 

 the chalk, and throughout its course of ten miles to the Thames is never 

 known to freeze. 



The observations of temperature on the Wandle by Mr. Bayard once 

 a week for 1889 are published by Mr. Thomas Gushing, F.R.Met.Soc. in 

 Report of the Meteorological Sub-Committee of the Croydon Micro- 

 scopical and Natural History Club for 1891. Mr. Cushing gives the 

 following abstract of the extreme temperatures observed and the yearly- 

 range : — 



