The Bakerian Lecture. An account of experiments to determine the amount of the dip of the magnetic needle London, in August 1821; with remarks on the instruments which are usually employed in such determinations

After describing the imperfections of the instruments in general use for ascertaining the dip of the magnetic needle, and adverting to the consequent inaccuracy and insufficiency of the observations made with them, Captain Sabine gives an account of the form of dipping-needle which he preferred for his experiments, and which was constructed for him by Mr. Dollond, upon principles laid down by Professor Meyer, of Gottingen. He then enters into minute details of the mode of pursuing and verifying his observations, the results of which, gained by three different methods, are as follows: viz. by 10 experiments with Meyer’s needle, 70° 2'.9; by the times of oscillation in the magnetic meridian, and in the plane perpendicular to it (mean by three needles), 70° 04'; by the times of vertical and horizontal oscillation, 7° 02'.6. So that 70° 03' may be considered as the mean dip of the needle towards the north, in August and September 1821, within four hours of noon, being the limit within which all the experiments were made. Alluding to former observations for the purpose of determining the dip in London, the author observes that, independent of any imperfection in the instruments, they were made in houses in close built parts of the metropolis, and, therefore, all subject to the influence of local attraction; and, moreover, that the correction found by observing the difference of the dip on the outside of the house cannot be regarded as an effectual remedy, inasmuch as the needle may still have been attracted by iron in the adjoining houses, or in the neighbourhood. It is, indeed, only requisite to try needles in different situations in a city, to be convinced how little dependence should be placed in the accuracy of such results: the author thinks that it is rather owing to this cause than to instrumental error, that the dip at the Apartments of the Royal Society is stated in the Philosophical Transactions for the present year to be 71° 06'. To avoid this source of error, Captain Sabine conducted the observations which form the subject of this lecture in the nursery-ground in the Regent’s Park, a situation which he regards in all respects eligible, and far removed from the influence of iron.