^32 Sir D. Brewster on certain Isothermal Lines in America. 



fore, the position of Jloulouk at 168° 20' west longitude, and 

 53° 53' of north latitude ; and from these data we shall 

 find D = 33°23' 



And the calculated mean temperature of Jloulouk 4'3°-980 

 Observed mean temperature 40 '325 



Difference .... +3 '655 

 This difference between the formula and observation is 

 much greater than usual ; but we shall presently see that it 

 must anse either from the observations not affording a correct 

 mean, or from the temperature of the place being affected by 

 local causes. 



I presume that Sitka is the same place as the Isle of Sitka, 

 iu the Great Northern Ocean, where Dr. Erman made his 

 magnetical observations. The following extract from Dr. 

 Erman's table, given in his letter to the academician M.Wis- 

 niewsky, and published in the Bulletin Scientijique, will enable 

 us to approximate to the position of Sitka. 



North Lat. West Long. 

 Nov. 4. In the Great Northern Ocean 56°54'-20 223°53'-20 



— 12. At the Isle of Sitka 57 3-12 



— 20. In the Great Northern Ocean 54 26*50 22122-80 

 As Dr. Erman has omitted to give the longitude of Sitka, 



we may infer from the preceding table that it cannot be far 

 from 222°. 



Hence we obtain D = 25° 38', and 

 The calculated mean temperature of Sitka . . 33°'84Fahr. 

 Observed mean temperature 45 '05 



Difference -11-21 



This difference is so extraordinary, that we must either have 

 mistaken the locality of Sitka, or there must be some singular 

 source of heat in the island, or some inexplicable error in the 

 observations. The reader will observe, that the difference is 

 now — , whereas it was + in the case of Jloulouk. 



Without taking the formula as our guide, we have only to 

 consider that Jloulouk, in latitude 53° 53', has only a tempera- 

 ture of 40^; while Sitka, in latitude 57° 3', has a temperature 

 of 45° ! in order to be convinced that the formula will not 

 be found to be in fault. We must wait, therefore, for other 

 observations from these regions before we can explain the 

 cause of this singular discrepancy. 



vi<4>AtiA 



