56 EIGHTH RKPORT — 1838. 



ai'cs differing very widely from each other, and distributed ge- 

 nerally round the whole circle, — and if the results in such case 

 accord well with each other, and with those of the unweighted 

 needle, — it may be concluded that tliere is no disturbing influ- 

 ence in the limb. 



Those who are desirous of making accurate observations, 

 should regard the preliminary examination of the axle and limb 

 of the apparatus they employ as an indispensable precaution. 

 When these points have been satisfactorily examined, and the 

 instrument is found correct, the natural magnetic direction, 

 both in regard to azimuth and inclination, is the most advan- 

 tageous for the observation of the dip. It is in the preliminarj^ 

 examination, that the method devised by Mayer, and that of 

 varied azimuths, are chiefly valuable*. 



It may now be satisfactory to exhibit the observations that 

 have been made at Westbourne Green in the years 1837 ^"d 

 1838 with different circles and approved needles. (Table III.) 

 The greater part of these instruments were made by Mr. Robin- 

 son since his attention has been particularly directed to the cir- 

 cumstances above noticed; and those who will take the trouble 

 to compare their performance with that of the several needles 

 employed by Captain Ross at the same station in 1835, 

 will have an opportunity of judging how great an improve- 

 ment has been effected in our English dipping needles since 

 that period. Of the two other instruments not made by Robin- 

 son, one was made by Gambey for Captain Fitz Roy, of the 

 Royal Navy, and kindly placed by that officer at my disposal, to 

 be employed in the observations in this report. The excellence 

 of the dipping needles of this artist is too well known to need 

 any comment in this place. The other instrument was made by 

 Mr. Thomas Jordan of Falmouth, the artist employed by Mr. 

 Fox to make the dip apparatus on the construction which he 

 has devised, and which is described in a paper in the 3rd vol. 

 of the "Annals of Electricity, &c." Mr. Fox's needles donoti-est 

 on a C5'lindrical axle supported by planes, but the axle is ter- 

 minated by exceedingly fine and short cylindrical pivots, which 



* The needle employed by Sir Everard Home in the observations published 

 in the last volume of the Phil. Trans. 1838, Part 2, appears, by its results at 

 the Athenaeum at Plymouth, and at Ham, near London, to have given dips ex- 

 ceeding the truth by about half a degree. It is probable that a careful examina- 

 tion would trace this error to imperfection in the axle; and in such case errors 

 of a contrary character would exist when the axle should rest on some other 

 points of its circumference, and may have influenced the determinations at 

 some of Sir Everard's foreign stations. By the methods pointed out in this 

 report, a table of errors at diflerent dips might be formed for this needle, by 

 which its results might be' corrected. This additional trouble would be well 

 bestowed in perfecting this extensive series, on which so much pains have 

 already been expended. 



