326 - Scientific Memoranda. 



plish the object of propelling water, by means of an instrument 

 having the form of an eccentric curve, resembling the spiral of 

 Archimides, made to revolve on an axis. The resistance offered 

 to the fluid in v^^hich it is immersed, results from the different dis- 

 tances of the two ends of the spiral propeller, from the axis. This 

 propeller acts in a box, having also a somewhat spiral shap^, 

 and the space between the two ends of the spiral, after describ- 

 ing one turn, is open to allow of the exit of the water driven out 

 by the propeller. The bottom of the box has a circular aper- 

 ture, of which the radius is equal to the shorter end of the pro- 

 peller from the axis. The water within this circle meets with 

 no resistance until it arrives at the line joining the two extremi- 

 ties of the propeller, when it is immediately acted upon by the 

 eccentric curved surface of the propeller. — Philosoph. Mag. 



On the errors in the course of vessels occasioned by local attraction^ 

 zoith some remarks on the recent loss of his majesty^ s ship, Thetis ; by 

 Peter Barlow, Esq. F. R. S. 



This paper was read before the Royal Society. The author 

 observes, that the errors arising from the deviation of the com- 

 pass produced by the attraction of ships, were formerly much less 

 considerable than at present, from the comparatively small quan- 

 tity of iron existing in the vessel. The increase of this disturb- 

 ing force in a modern ship of war, is easily accounted for bj the 

 immense proportion of iron now employed in its construction, by 

 the use of iron ballast and iron tanks, of iron knees, iron cables, 

 and above all, of iron capstans, besides various other articles 

 made of the same materials, forming altogether a very large and 

 powerful magnetic mass. 



The direction and intensity of the deflecting forces thus pro- 

 duced, vary in different latitudes and on different sides of the 

 equator ; being greatest in the highest latitudes, where the dip 

 is considerable, and when the ship's course is east or west : and 

 in high southern latitudes, being the reverse of what it is in high 

 northern latitudes. In his majesty's ship Gloucester, which n^ay 

 be taken as an example, the deviation of the compass in the east 

 and west points was found to be, in the British channel, nine 

 deg. thirty min. ; so that after running ten miles, the vessel 

 would be more than a mile and a half to the southward of her 

 reckoning ; and so on in proportion as the distances increased. 



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