STEAM NAVIGATION. 



before it had yet attained all its present degree of perfection, it 

 was in extensive operation under the direction of the General 

 Screw Shipping Company. Seven vessels belonging to that com- 

 pany were in operation during the twelve months ending 31st 

 December, 1849, during which time they performed 170 voyages, 

 being an average of about 24| voyages per vessel. The total dis- 

 tance run was 110849 geographical miles, being at the average 

 rate of 15835 miles per^vessel, and about 648 miles per voyage. The 

 average speed was 8 to 8|- geographical miles per hour, and only one 

 casualty, and that one in the Thames, occurred during the year. 



The speed of the best and most recent of these vessels in still 

 water, running the measured mile in the long reach of the Thames, 

 was found to be 9 '68 knots per hour. 



78. Practical authorities have suggested, that the greatly in- 

 creased and rapidly increasing number of screw ships running 

 between the British and American ports, suggests the expediency 

 of a revision of the post-office contracts, with a view to public 

 economy, without any real sacrifice of efficiency. It is considered 

 that no difference of time worthy of consideration now prevails 

 between the passages of the mail-packets and the screw- vessels ; 

 but even admitting a difference, it is certainly not so great as that 

 which exists between the speed of the mail and that of the 

 express trains on railways. If then the mail contracts on the 

 iron lines are sufficiently well performed by the trains of second- 

 rate speed, why may not the like contracts on the lines of water 

 be similarly executed, where the difference of cost would be 

 enormous, and the difference of speed comparatively insignificant. 



It is obvious that these observations are applicable not only to 

 the lines of steamers which carry the United States and Canadian, 

 but also to the West Indian, and in a word, to all the ocean lines. 



79. But when screw propulsion is used, a much greater velocity 

 of revolution is required to be given to the screw-shaft, a much 

 greater number of revolutions per minute being necessary, than 

 the greatest number of strokes per minute made by any steam- 

 engine of the common construction. It was necessary, therefore, 

 in adopting screw propulsion, either to provide expedients by which 

 the velocity of rotation of the screw-shaft shall be greater than 

 that of the crank-shaft, in the requisite proportion, or to modify 

 the form and proportions of the steam cylinders and their appen- 

 dages, so that the number of strokes per minute should be aug- 

 mented, so as to be equal to the necessary number of revolutions 

 per minute of the screw- shaft. 



Both these contrivances have been adopted by different con- 

 structors. Engines constructed on the former plan are called geared 

 engines, and those constructed on the latter direct acting engines. 

 166 



