108 



if perforated and containing deep cavities, would soon have absorbed 

 her atmosphere, supposing she ever had one. 



The author's letter concludes with some remarks on Euler's for- 

 mula for the rolling pendulum, from which the perfect accuracy of 

 Laplace's theorems for the length of the convertible pendulum roll- 

 ing on equal cylinders may be inferred, without any limitation of 

 their magnitude, or of the form of the pendulum. It also affords the 

 proper correction for the arc of vibration. 



On the Anomaly in the Variation of the Magnetic Needle as observed 

 on Ship-board. By William Scoresby, jun. Esq. Communicated 

 by the Right Hon. Sir Joseph Banks, Bart. G.C.B. P.R.S. Read 

 February 4, 1819. [Phil. Trans. 1819, p. 96.] 



The anomalies in magnetic observations on ship-board were usually 

 attributed to imperfections of the azimuth compass, till Capt. Flinders 

 suggested the influence of the iron used in the construction of the 

 ship as their probable source, a suggestion since confirmed and illus- 

 trated by Mr. Bain. 



In this paper, Mr. Scoresby has given a table of the selected re- 

 sults of his observations relating to this subject, conducted on the 

 coast of Spitzbergen, in the years 1815 and 1817. To these he has 

 added some general inferences, deduced at the time of observation, 

 and observes, that the anomaly of attraction is probably the greatest 

 in men-of-war, and ships containing large quantities of iron, though 

 it also exists to a considerable extent in merchantmen where iron 

 forms no part of the cargo, especially in high latitudes where the 

 dip of the needle is great. 



On the Genus Ocythoe ; being an Extract of a Letter from Thomas 

 Say, Esq. of Philadelphia, to William Elford Leach, M.D. F.R.S. 

 Read February 4, 1819. [Phil. Trans. 1819, p. 107.] 



After describing a genus of Ocythoe, which the author regards as 

 new, he observes that the Ocythoe offers a deviation from the ordi- 

 nary laws which apply to the testaceous Mollusca, inasmuch as it 

 resides only in the last volution of the shell ; and as the shell does 

 not fit the body, it appears probable that it was not made for it, 

 more especially as there is no attachment between the shell and any 

 part of the body. The shells that approach nearest to Argonauta are 

 of that order : this supposition, however, is not corroborated by the 

 habits of the animal ; for all hitherto discovered of that order swim 

 to the surface ; and having no other organs of locomotion than fins, 

 cannot glide upon the bottom. We must, therefore, suppose this to 

 have been the habit of the animal ; and yet it is hardly admissible 

 that in that case it should have eluded the observation of voyagers, 

 when the shell has often been found occupied by the parasite. 



