Tempeeatuee Coefficient of the Balance Magnet. 



xlix 



Mondays ; the remaining five-sixtlis are of the readings on days with those on the 

 days immediately following : — 



1844. 123 Comparisons, 3' =: 8'33. Approximate value of 5', by temperature experiments and vibrations, =z 3*8 



1845. 126 Comparisons, g'= 8-87 = 3-0 



69. The most severe method of testing the accuracy of the coefficient thus 

 obtained, is that of artificially heating the Observatory, and of comparing the in- 

 strumental readings on days when so heated with those on days when the tempera- 

 ture of the magnet depends on natural causes ;* the results of such a series of com- 

 parisons are given below. In order to render the series perfectly trustworthy, all 

 selection has been avoided. From January 1 till March 16, 1843, the stove in 

 the Observatory was lighted every day, with the exception of January 23 ; it was 

 not lighted on any other days of the year, excepting March 22 and 23, October 19, 

 November 25, and December 21 ; the means of the instrumental readings, corrected 

 for temperature by the adopted coefficient (7"9), on each of these days were compared 

 with the means on the four or five days immediately preceding or succeeding, and 

 the means on March 13, 14, and 15 (the last three days on which the stove was 

 continuously lighted), were compared with the five succeeding days on which the 

 stove was not lighted. The sums of the differences 2 a (t) and 2 a (y) are given in 

 the following Table, together with the sums of the differences of the external tem- 

 peratures 2 A (T) for the same days : — 



Table 18. — Results of Comparisons when the differences of the Temperature 



depend on Artificial Causes. 



Day of 

 com- 





Days compared with 



No. of 

 compa- 



2A(T) 



2A(0 



2A(y) 



Error of 

 coefficient. 



Resulting 

 coefficient. 



parison. 







risons. 













1843. 





1843. 





o 



„ 



Mic. Div. 



Mic. Div. 



Mic. Div. 



Jan. 23 



Jan. 



18—21, 24—28 



9 



-f-63-4 



139-2 



+ 195-7 



+ 1-41 



6-49 



Mar. 13 



Mar. 



16—21, 



5 



-21-2 



35-4 



+ 85-7 



+ 2-42 



5-48 



Mar. 14 



Mar. 



16—21, 



5 



-27-5 



31-4 



-i- 41-2 



+ 1-31 



6-59 



Mar. 15 



Mar. 



16—21, 



5 



-29-0 



31-9 



+ 28-7 



+ 0-90 



7-00 



Mar. 22 



Mar. 



16—21, 24—28 



9 



+ 79-1 



58-8 



+ 6-5 



+ 0-11 



7-79 



Mar. 23 



Mar. 



16—21, 24—28 



9 



-f55-7 



80-4 



- 58-3 



-0-73 



8-63 



Oct. 19 



Oct. 



14—18, 20—25 



9 



-41-2 



108-3 



-278-4 



-2-58 



10-48 



Nov. 25 



Nov. 



21—24, 27— Dec. 1 



9 



-84-5 



1131 



+ 75-5 



+ 0-67 



7-23 



Dec. 21 



Dec. 



16—20, 22—27 



9 



-49-9 



990 



+ 7-4 



+ 0-07 



7-83 



The results of all the comparisons are — 



2 A (T) = - 55°-l, 2 A (0 = 697°-5, S A (y) = + 104-0 mic. div. ; error of adopted coefficient = + 0-15 mic. div. 



* This method was that first tried for the determination of the coefficient, but ultimately abandoned 

 on account of the heated stove generating currents of air in the room and magnetometer boxes. 



MAG. AND MET. OBS. 1843. n 



