18 



CUANGES OP THE MAGNETIC DECLINATION 



number of hourly readings (equal to 17), and m the mean disturbance, then m = 



+ f^:^^ This quantity is analogous to the mean error of an observation. In 



\h — 1 



the following comparisons we must always bear in mind that the observations for 



the present discussion arc rather limited, and that the comparisons with results at 



Lake Athabasca and Fort Simpson are of a date nearly ten years earlier. This 



interval is perhaps favorable to the comparison. 



At Van Rensselaer Harbor the mean disturbance force is greater than at either 



place just named, and pretty regular during two well-marked periods, as shown by 



the following table : — 



Table of the Mean Distitrbance of the Declination at Van Rensselaer IIarbor, taken 



WITHOUT regard TO DIRECTION, FOR EACH OF THE OBSERVATION HOURS, AND EXPRESSED IN MiNUTES 



OP Arc. 



Local Mean Time. 



The disturbing force is least during the day (if such an expression is admissible 

 in this case), from 10 A.M. to 7 P. M., and greater and equally regular during the 

 hours of the night {I), from 8 P. M. to 8 or 9 A. M. At Lake Athabasca the hours 

 of least disturbance are between 9 A. M. and 7 P. M., and at Fort Simpson from 

 10 A. M. to 7 P. M. Captain Lefroy, in his discussion of the disturbances of the 

 declination remarks : " There are indications in each of the three curves (for Lake 

 Athabasca, Toronto and Sitka) of a small increase in the mean disturbance about 

 noon." At Van Rensselaer Harbor we find the maximum disturbance at this very 

 hour preceded and followed by quite small A^alues ; this circumstance certainly 

 deserves our particular attention. Further coincidences of the disturbing force can 

 be noticed at 5 P. M., at which hour at Van Rensselaer, Lake Athabasca, and Sitka 

 the minimum disturbance has been observed. At Fort Simpson, in April and May, 

 1844, the mean disturbance was but one-fourth of that observed in January, Feb- 

 ruary and March at Van Rensselaer, and thie ratio of the minimum to the maximum 

 value was 5.6 and 2.0 at the two places respectively. 



By adding the squares of the differences for each hour of the day and month, we 



find the mean monthly disturbance by the formula \^ -. The mean disturb- 



^ ■ •' \iV— 24 



ance for each month is as follows : — 



In January, 1854 ±30' 



In Februaiy, " ±65 



In March, " ±40 



' rrinfii)iilly due to a very large disturbance. 



