Veetical Component or Magnetic Force. 



373 



Note on the least probable error of an observation of the horizontal component of magnetic force.— 

 Investigations for the probable errors of observation corresponding to those for the magnetic declination, have 

 not been made for the horizontal component ; it ivill not be difficult, however, to approximate to the least 

 probable eiTors of an observation from the values of the mean difference. The mean difference of an 

 observation is least in the month of January, and in the months of June and July, being 0*0003-5, 0-00040, 

 and 0-00047, for the three months respectively ; which, if the distribution of the errors follows nearly the same 

 law as that found for the magnetic declination, will give probable errors of about 0-00024, 0-00027, and 

 0-00032, of the horizontal component. 



The mean difference, for the year, of an observation of the horizontal component, is a minimum about 

 6'' A.M., and l"" and 2^ p.m., being about 0-00050 ; the probable error is about 0-00034. 



In order that an observation of the horizontal component should have the least probable error, it should 

 be made in the months of January, June, or July — and early in the morning. The least probable errors are 

 about 2'' A.M. in these three months, being less than two ten-thousandths of the whole horizontal component. 



Vertical Component op Magnetic Force. 



TABLE XXXVIII. — Mean Values of the Variations of the Vertical Component of Magnetic Force, the 

 whole Vertical Component being Unity, for each Civil Week-Day, Week, and Month of 1844. 



Table XXXVIII. has been formed from the daily means in micrometer divisions by the following formula : — 



f=n 0-00001, 

 where / is the quantity in the Table, and n is the daily mean in micrometer divisions corrected for temperature. 



MAG. AND MET. OBS. 1844. 5 B 



