344 Sir D. Brewster on the Law of Daily Temperature. 



larity to the isodynamical magnetic curves*, we are disposed 

 to view so remarkable a phenomenon as the result of a phy- 

 sical condition of the earth itself, and produced by causes 

 connected with its magnetic, or galvanic, or chemical agencies. 



Before we can determine, with anything like accuracy, the 

 form of the isothermal lines, we must, in reference at least to 

 past observations, be able to deduce from them the mean tem- 

 perature of the places at which they were made ; and this can 

 only be done after we have determined the law of the distri- 

 bution of heat throughout the day and the year, the mean 

 annual orbit as it were, described by the summit of the mer- 

 curial column. Previous to the determination of the law of 

 distribution, we could not obtain the mean annual temperature 

 even from three daily observations, and still less from two or 

 one observation. The mean of the maximum and minimum fur- 

 nished us with an approximation ; but as the instruments with 

 which these extremes were measured were continually going 

 wrong, the results which they yielded were always liable to 

 error. 



In order to solve this problem, I suggested to the Royal 

 Society of Edinburgh the plan of establishing hourly observa- 

 tions at Leith on the east coast of Scotland. They were con- 

 tinued for four years, and yielded results of the most unex- 

 pected kind ; but the observations for the first two years only, 

 viz. for 1824 and 1825, have been published. We shall 

 therefore in general confine ourselves to the results to which 

 they lead. The first of these is the determination of the hours 

 of mean temperature for the whole year, and the interval which 

 elapses between these hours, which has been called the Critical 

 Interval. 



Mean 9 11$ ... 8 26 ... 11 14f 



Now it is a remarkable circumstance, that at Padua the 

 critical interval is 11 hours 14 minutes; a£ Appenrade 



* This remarkable fact, which we first pointed out in the Edinburgh 

 Transactions, vol. ix. pp. 223, 225, is fully admitted by M. Erman. "The 

 thermometrical observations at Jakousk," says he, " confirm, in a remark- 

 able manner, the relations which have been discovered between these tem- 

 peratures and terrestrial magnetism ; for at this place we arrive at once 

 in the neighbourhood of the meridian of one of the magnetic poles, and in 

 the meridian of the greatest cold which is known in the whole world."— 

 Reise urn die Erde, torn. ii. p. 250. See also the Encyclop. Brit. art. Mag- 

 netism, vol. xiii. p. 695. 



