26 MEMOIR OF DR. DALTON, AND 



mentioned. Wargentin, Halley, and Celsius had all observed 

 the action of the aurora on the magnetic needle. Dr. Halley 

 had supposed it to be caused by magnetism. Dalton went 

 more fully into the subject than his predecessors, without, 

 however, taking all difficulty from it. In this treatise we see 

 an instance of the pertinacity with which he held ideas which 

 he had formed. But we find him altering his opinion on the 

 height of the aurora; his observations led him to believe 

 the height to be about 150 miles; afterwards he considered 

 it to be about 100. Numerous as have been the attempts 

 to ascertain the height, the differences ranging from feet to 

 thousands of miles, Dalton still, in 1834, severely criticised 

 all the observations which differed greatly from his early 

 results. To this treatise on meteorology he added little, 

 although a new edition appeared after forty years. He 

 then says, that it is printed verbatim (adding only a small 

 appendix), "as I apprehend it contains the germ of most of 

 the ideas which I have since expanded more at large in dif- 

 ferent essays, and which have been considered as discoveries 

 of some importance." But he says also, that " the subject 

 here treated of appeared to the author to be very imperfectly 

 appreciated, or little understood, by some of the modern 

 writers on meteorology," and it is probably true that the 

 facts and theories he advanced had, in some or many in- 

 stances, been worked out by others with little aid from his 

 book, because, although occasionally quoted, it was really 

 very little known. This arose from a peculiarity in his mode 

 of publishing it. It was like all his books printed for him- 

 self, and was never allowed to make its due way in an inde- 

 pendent manner among the booksellers, nor had the essays 

 the advantage of being read to a society, or given out by 

 any journal. 



