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^CXXIV. Of the Variations of the Temperature of tJie 

 Summer and JJlnter Seasons that take place in different 

 Years. Bi/ Righabd Kirwan, Esq. LL.D. F.R.S* 

 and P.R.LA* 



JL o reason with precision on this Subject, we must at first 

 abstract from ail sublunary phvsical causes, and indicate 

 the temperature appropriated to difterent latitudes from mere 

 astronomical considerations. 



Hallev has ingeniously resolved this problem so far as 

 the mere ratios of heat in the different seasons are con- 

 cerned. (Phil. Trans. Abr. ii. p. 163 ; and Lambert in his 

 Pyrometric, § 596. 



Hallev, calculating the ratios of heat communicated by 

 the sun to the earth (which he considers merely as a pla- 

 net, abstracting from all distinction of land and water,) in 

 the different seasons in the northern hemisphere, reduces 

 these seasons to three, the equinoxes, the summer solstice, 

 and the winter solstice ; and attending only to the sines of 

 incidence of the sun's ravs, and the duration of their action, 

 he sets the heat coinmunicated at latitude 0, on the days 

 of the vernal and autumnal equinoxes, at 20,000 ; and on 

 'the tropical days in the same latitude at 18,341 : and then 

 adds the ratios which the heat in every 10th degree of norlh 

 latitude bears to these at the same periods. Lambert adds 

 the ratios of lat. 49° and 66° 33', stating the equatorial 

 heat on the equinoxial day at 999. 



But to express these ratios in thermometrical measures 

 we must endeavour to find the oreatest heat of the equi- 

 noxial day, taking a mean of the heat of the morning at 

 two o'clock, and the evening under the equator, or very 

 near it ; and this I find to be 68° or 89° of Fahr. (see Ulloa, 

 Mem. Philo^;oph. p. 61,) in the northern hemisphere, on 

 the 20th of March, on the ocean ; to which, indeed, we 

 must confine ourselves in this inquiry, and particularly the 

 Atlantic, for no uniformity can be expected on land. 



It is uncertain what thermometer of Reaumur Ulloa 

 employed, whether the true or the false; and hence I place 

 the heat at 8S° of Fahr. 



This correspondence being found, the thermometrical 

 degrees corresponding with all the other ratios are easily 

 found by the rule of proportion, and the degrees thus found 



* From his paper entitled '• Of the Variations of the Atmosphere. 

 j8oi." 



I cdl 



