xlviii 



Introduction to the Makerstoun Observations, 1843. 



ture experiments it is 4*9 micrometer divisions ; the mean of tlie two series in 1844 

 gives q' — 7*91, while the temperature experiments give it 3*8 microm. div. 



%b. The following Table contains the results deduced in three cases when re- 

 spect has been paid to the sign of t^ — t^^^ ; the last column contains the value of q\ 



obtained by giving equal values to the quantities 



2 A (0 



and 



- 2 A (/) 



Table 17. — Results of Comparisons, regarc 



being paid to the si 



gn of tp 



-tp+r' 



Period. 



P / ^p + r 



*P \ ^p+r 



Mean 

 Value of 



2A(0 



-2A(3/) 



-2A(y) 



-2A(0 



2A(y) 



2A(y) 



2A(0 



-2A(0 



1843. 



o 



Mic. Dir. 



Mic. Div. 



o 



Mic. Div. 



Mic. Div. 



Mic. Div. 



June 1 — July 22 



4003-4 



33523-8 



8-37 



19518-9 



163340-7 



8-37 



8-37 



1844. 

 May 9— May 24 



5404-3 



37559-9 



6-95 



3011-1 



29061-8 



9-65 



8-30 



Aug. 3— Sept. 18 



6409-0 



52022-7 



8-12 



16295-4 



134950-6 



8-28 



8-20 



For all the periods 



15816-7 



123106-4 



7-783 



38825-4 



327353-1 



8-431 



8-107 



66. The three final results obtained are q' = 7-832, q' = 7*903, and / = 8-11 ; 

 the adopted value of q' = 7'9 micrometer divisions. 



67. Early in the investigation of this subject it occurred to me that it might 

 not be desirable to eliminate all the effects of change of temperature on the posi- 

 tion of the needle, as the actual daily or diurnal variations of the earth's magnetism 

 might depend to some extent on the same cause. The results, obtained in many 

 ways which it is not necessary to repeat wholly here, shewed that, at least for periods 

 of a month, no such connexion exists, or that it is inappreciable. To such an extent 

 had the supposition been refuted, that it was ultimately totally forgotten by me, 

 and, of course, it was also forgotten that others might entertain a similar suspicion ; 

 the fact that it has been proposed as an objection to the method will render it de- 

 sirable that I should adduce distinct evidence of its futility. 



68. As it will scarcely be supposed by any one that the earth's magnetism 

 varies immediately with changes of the aerial temperature, the results obtained from 

 the comparison of one day with the next following may be first considered. As far 

 as the results. Table 15, go, the comparisons of one day with the next following 

 give a considerably larger value of q' than that adopted, and, therefore, one differ- 

 ing still more from the value obtained by the usual method ; this at least tends to 

 prove that the result adopted is much better than that by the other method. In the 

 following, which also prove the same fact, one-sixth of the comparisons are of the 

 readings on days with those on the second days following, namely, Saturdays with 



