232 



of the divisions of the scale was 3'.98 ; with the modifications 

 since introduced into the reading part of the apparatus, the 

 scale 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 from (7) that 

 the changes of inclination are inferred witli 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 in- 

 clination 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 pro- 

 jected results of the observations of each of these weeks, 

 together with that of the mean of the whole. An inspection 

 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 

 discordances being only such as are due to the known irre- 

 gularities in the direction of the earth's magnetic force. 



A communication from the President was read, contain- 

 ing some remarks supplementary to the account which he 

 had given at a former meeting, of his Researches respecting 

 Fluctuating Functions, (see Proceedings, June 22nd, 1840). 



degree, with the minute changes induced by the variations of the earth's force. 

 Jt remains for future examination to determine how far such permanent changes, 

 if they occur, may impair the accuracy of the lesults. 



