1825.] Philosophical Transactions for 1825, Part L 459 



anomaly in the directions of the needle in the two cases, which 

 was observed by Mr. Barlow and myself, arose from the cause 

 which I have assigned for it in my former paper ; namely, the 

 difference in the changes of temperature in the magnets when 

 in doors and when in the open air. 



*' The diurnal changes in the terrestrial magnetic intensity 

 have been determined by Professor Hansteen, by means of the 

 vibrations of a needle delicately suspended. From these obser- 

 vations it appears, that in general the time of minimum intensity 

 was between ten and eleven o'clock in the morning ; that the 

 maximum happened between four and seven for the month of 

 May, 1820, and about seven o'clock in the evening for the 

 month of June. The intensity which, in these observations, is 

 taken as unity, is that deduced from an observation made during 

 an aurora borealis ; but for the purpose of comparison, I have, 

 for the months of May and June, taken the intensity deduced 

 from liis observations at 10^' SO"" in the morning as unity, 

 reduced the intensities, which he gives for other times in the 

 day, to this standard, and placed them in the following table, 

 with the corresponding intensities deduced from my own obser- 

 vations. 



'* The principal difference to be observed in the nature of the 

 changes of intensity during the day, in the two cases, is, that 

 from my observations, the intensity appears to decrease more 

 rapidly in the morning, and increase more slowly in the after- 

 noon, than it does from those of Professor Hansteen ; but the 

 general character of these changes is as nearly the. same ^^s we 

 can expect from methods so different, at different times, and at 

 places where both the variation and dip of the needle are 

 different. My object however was, to point out what might be 

 deduced from a series of such observations as 1 have detailed, 

 rather than to compare the results deduced from them with 

 those obtained by others, for which purpose it would have been 

 necessary to have continued them for a greater length of time. 



^' We have seen that with the magnets I made use of, their 

 intensity being nearly 218 M, at the temperature 60^, a change 

 in their temperature of 1° would cause a change of intensity ojf 



