327 



ivory circle. The needle, previous to beginning to count the vibra- 

 tions, was drawn 50 or 60 from the magnetic meridian by another 

 needle, and left to oscillate. When it had reduced its arc of vibra- 

 tion to 30, the counting of its vibrations was commenced, and ter- 

 minated at 5. It usually took between 300 and 400 vibrations to 

 reduce the arc of vibration to this limit, occupying from 1 2 to 16 

 minutes. Four of the needles, with an apparatus in duplicate, were 

 sent to the author from Professor Hansteen of Christiana, to be em- 

 ployed in comparative experiments in various parts of Great Britain. 

 They were vibrated in Edinburgh by Captain Basil Hall and Lieut. 

 Robert Craigie, and the results are set down with the rest in this 

 paper. The needles being returned, were also used in the experi- 

 ments between Paris and London. The two remaining needles were 

 made by Dollond, of the same size and form as Professor Hansteen's. 

 The author then relates his experiments, which were made on the 3rd 

 of December, about seven weeks previous to his departure for Paris, 

 in the garden of the Horticultural Society at Chiswick ; and on the 

 15th of January, atThornfold Park, near Tunbridge Wells ; and on the 

 30th of January, in the garden of the Royal Observatory at Paris. 



An opportunity occurring, three of the needles were sent to En- 

 gland early in April, and, with one sent by Captain Hall from Edin- 

 burgh, were vibrated by Captain Chapman, R.A., in the garden of 

 the Horticultural Society, and returned to Paris. 



Professor Hansteen's four needles were always kept separate, but 

 those by Mr. Dollond together, and nearly in contact. To try the 

 effect of separation, these were separated from the 14th of March to 

 the 30th of April, and being then again tried in the same place as 

 before, their times of vibration were found unaltered. 



The author then states, in the form of an abstract, the results of 

 the several experiments, all the details of which are subjoined in the 

 form of tables. The mean of all gives a ratio of horizontal directive 

 force in Paris greater than in London, in the ratio of 1068 : 1000 ; 

 and on the supposition that the dip in London is 69 45', and in Paris 

 67 58', the ratio of the intensity of directive force on the dipping- 

 needle comes out greater in London than in Paris, by about 15 parts 

 in 1000. 



On the Resistance of Fluids to Bodies passing through them. By James 

 Walker, Esq. F.R.S.E. Communicated by Davies Gilbert, Esq. 

 M.P.V.P.R.S. Read May 31, 1827. [Phil. Trans. 1828, p. 15.] 



The object of this paper is to explain a new mode of measuring the 

 resistance of fluids, which has of late become more than formerly an 

 object of research owing to the introduction of steam navigation. 



The resistance of a fluid per se, is, theoretically speaking, as the 

 square of the velocity ; but, independently of friction and viscidity, 

 this theory is only applicable to the case of a body entirely and deeply 

 immersed. If it float on the surface (as a boat), the elevation of the 

 water in front, and its depression behind, disturbs the exactness of 



