230 ON A NEW MAGNETICAL INSTRUMENT. 



divisions have nearly the same value as in the instrument for the 

 measurement of the declination, so that the readings may be 

 made with certainty to less than the tenth of a minute. The 

 present value of the inclination in Dublin is about 70 50'; and 

 the mean deflection produced by the iron bar in its actual position 

 being about 19, it follows that the changes of inclination are 

 inferred with the same degree of precision, very nearly, as the 

 observed changes of angle. 



The last test to which the instrument was subjected was, to 

 employ it for some time in the regular observation of inclination 

 changes, for which it is destined, and to ascertain how far the 

 mean results of the observations of successive weeks agreed in 

 exhibiting the law of the diurnal variation. The instrument was 

 accordingly observed for five successive weeks, every second hour 

 during the day and night, and the means calculated, omitting those 

 days in which the series was broken by changes of adjustment 

 during experiment. The curves now laid before the Academy 

 represent the projected results of the observations of each of these 

 weeks, together with that of the mean of the whole. An inspec- 

 tion of them is sufficient to show that the curves of the separate 

 weeks accord with one another, and with the mean, as nearly as 

 can be expected in the results of such limited series, the dis- 

 cordances being only such as are due to the known irregularities 

 in the direction of the earth's magnetic force. 



