1 Introduction to the Makeestoun Observations, 1844. 



result is the mean of 8 values of q, obtained from comparisons of the mean readings 

 of the balance magnetometer upon about 190 days in the months of May, June, July, 

 August, and September 1844 and 1845 ; it is extremely probable that the tempera- 

 ture coefficient for the balance magnetometer is constant for the ordinary tempera- 

 tures of observation. 



77. As it was found impossible to determine k the value of one micrometer 

 division in parts of the whole vertical component, by means of the vertical vibra- 

 tions, the value of q obtained from hot and cold water experiments could not be em- 

 ployed, since the observations could not be reduced to parts of vertical force, nor 

 could the value of q be reduced to micrometer divisions. In consequence of this 

 difficulty, the method already described for the bifilar magnetometer was first em- 

 ployed for the determination of q the temperature coefficient in micrometer divi- 

 sions : the details of several of these comparisons will be found, pages xlv., xlvi., 

 xlvii., xlviii., and xlix., Introduction, 1843. It was found from these comparisons, 



1st, That the value of q, when a sufficient number of comparisons had been 

 obtained, was independent of the interval between the days compared. 



2d, That the value of q remained the same after various adjustments of the 

 needle ; the vertical screw for adjusting the sensibility never having been touched. 



3d, That the value of q has remained constant while the time of vibration in a 

 vertical plane has varied from upwards of 11 s to less than 6 s ; from which result it 

 has been concluded that the value of k also has been constant. 



4th, That the value of q is the same, whether the differences of temperature of 

 the magnet have been due to natural or artificial causes, and whether the differ- 

 ences of temperature of the magnet have had the same sign or an opposite sign from 

 those of the temperature of the air. 



78. From the 1st and 4th conclusions, it follows that the variations of the ver- 

 tical component of the earth's magnetism are independent of the temperature of the 

 air and of the temperature of the soil.* 



79. The mean of all the results in the volume for 1843, Introduction, pages 

 xlvii. and xlviii., gave 



q' = 7*90 micrometer divisions ; 



and adopting the value of k, obtained from deflections, No. 59, 



g = 0-000079. 



Which result is only -L more than that obtained from the hot and cold water ex- 

 periments : it appears in the case of the Makerstoun instrument that the errors of 

 the usual methods are found chiefly in the determination of k ; this, however, is not 

 always the case. 



The observations for 1843, 1844, 1845, and 1846, in micrometer divisions, have 

 been corrected by the value 



* See foot-note, p. 395 of the present volume. 



