60 Royal Society. 



force of the resistance due to the velocity is communicated through 

 a small pipe projecting into the water below the bottom of the ship: 

 this force, acting upwards, compresses a portion of enclosed air in a 

 small cylinder, which air communicating by means of a little pipe 

 with the bulb of a glass tube — bent like a common barometer — 

 raises the mercury in the tube, by depressing it in the bulb. 



But as any single column of water and air thus acting upon the 

 surface of the mercury in the bulb alone must depend not only upon 

 the resistance due to the velocity, but also upon the distance of the 

 cylinder from the water-line, which distance or height varies with 

 every sea, and alters more permanently as the draught of water 

 changes, a compensation was necessary ; and the inventor has found 

 one, which he considers perfect for all these variations, by applying 

 a second column ofioater and air to press upon the other surface of 

 the mercury, viz. that in the glass tube. This second column is 

 precisely like the first as regards the pipe and cylinder, and commu- 

 nicates with the sea by an aperture or apertures, presented in such 

 a direction that velocity does not produce any increase of pressure. 

 Thus the mercury in the indicator is placed between two columns 

 of water and air, which are always equal to each other in length, and 

 the mercury rises according to the difference between the pressures 

 upon its two surfaces, the result of resistance or velocity alone. 



The air-pipes may be conducted in any direction, and the indi- 

 cator, which swings upon gimbals, may be placed in any part of 

 the ship. The two water-pipes are conducted into one tube in the 

 bottom of the ship, divided into two separate chambers for the dif- 

 ferent forces. 



In addition to the speed, the true course or leeway of the vessel 

 is indicated upon a horizontal segment divided into degrees, over 

 which a needle is moved by a rod connected with the above-men- 

 tioned double tube ; and the whole is kept continually in the true 

 direction of the ship's motion by a float or vane attached to the 

 lower end of the tube in the water. 



Feb. 14. — " Supplementary Observations on the Structure of the 

 Belemnite and Belemnoteuthis." By Gideon Algernon Mantell, Esq., 

 LL.D., F.R.S., Vice-President of the Geological Society, &c. 



In this communication the author describes his recent investiga- 

 tions on the structure of the two genera of fossil Cephalopoda, whose 

 remains occur so abundantly in the Oxford clay of Wiltshire, 

 namely, the Belemnite and Belemnoteuthis, as supplementary to his 

 memoir on the same subject, published in the Phil. Trans. 1848. In 

 that paper evidence was adduced to show the correctness of the 

 opinion of the late Mr. Channing Pierce as to the generic distinction 

 of these two extinct forms of Cephalopoda. 



As however several eminent naturalists had expressed doubts as 

 to some of the opinions advanced by the author in his former memoir, 

 figures and descriptions are given in the present notice, of beautiful 

 and instructive specimens lately discovered in Wiltshire, and which 

 he conceives establish his previous conclusions. Dr. Mantell then 

 states as the result of his examination of several hundred examples, 



