130 THIRD REPORT — 1833. 



capable. This must be an objection, even where the observa- 

 tions are made in a fixed observatory ; but where an apparatus 

 has to be moved from one station to another, I think the method 

 could scarcely be applied successfully, principally on account 

 of the delicacy of the preliminary observations, and of the time 

 requisite for making them, in addition to that required for the 

 observations by which the terrestrial intensity and its variations 

 are to be determined. However greatly I may admire the saga- 

 city which Professor Gauss has shown in devising means for 

 the determination of an absolute measure of the horizontal in- 

 tensity, I cannot avoid seeing the difficulties which may occur 

 in its practical application. 



The method which Professor Gauss proposes, and has prac- 

 tised, of observing the course of the daily variation, and of de- 

 termining the time of vibration, by means of a plane mirror 

 fixed on the end of the needle, perpendicularly to its axis, and 

 observing the reflected image of the divisions of a scale by 

 means of a theodolite fixed at a distance, appears to admit of 

 the greatest possible precision, and will probably supersede 

 other methods of observing the daily variation. 



I have adverted to the necessity of careful and dihgent ob- 

 servation of all the phaenomena of terrestrial magnetism, as the 

 surest means of arriving at a knowledge of their causes : it is 

 with i-eluctance I state it, but I believe it to be a fact, that this 

 is the only country in Europe in which such observations are not 

 regularly carried on in a national observatory. Such an omission 

 is the more to be regretted, seeing that no one has, I believe, 

 cai'ried on a regular series of observations on the diurnal va- 

 riation, since the valuable ones by Colonel Beaufoy were inter- 

 rupted by his death, this interruption happening at a time when 

 it was peculiarly desirable that the series should be unbroken. 

 At this time the needle near London had begun to show a re- 

 turn towards the true meridian ; but whether this was one of 

 those oscillations which have occasionally been observed, or 

 that, having really attained its maximum of westerly deviation, 

 it was returning in the contrary direction, is, I believe, undecided 

 at the present moment. Of all the data requisite for deter- 

 mining the laws which govern the phaenomenon of the variation, 

 the time of the maxima and their magnitude are the most im- 

 portant. I trust that ere long the important desideratum will 

 be supplied of a regular series of magnetical observations in 

 the national Observatory of Great Britain. 



Royal Military Academy, 

 22nd June, 1833. 



