1B18.] Co?. Beaufoy on the Spiral Oar. 247 



and which Mr. Dick is of opinion might be usefully employed in 

 propelling ships of war. A contrivance of this kind 1 saw, 

 between 30 and 40 years past, in Switzerland, in the model 

 of a flat-bottomed vessel, brought by Monsieur Bosset from the 

 East Indies, but made in China ; this model had underneath its 

 bottom a spiral, which was turned when wanted with consider- 

 able rapidity by clock-work, put in motion by a spring similar 

 to a watch ; the vessel being plaeed in a tub full of water, the 

 spring wound up, and the helm put over, more or less according 

 as the tub was large or small, the boat continued running in a 

 circle until the clock-work went down. 



I witnessed an experiment on a much larger scale, made in 

 Greenland Dock by Mr. Lyttleton, formerly a master in the 

 Royal Navy. This gentleman had fixed to the stern-post of a 

 Virginia pilot-boat, a frame containing a large copper spiral, 

 which, by a winch, turned by two or more men, gave it a rotary 

 motion ; the effect was much less than expected ; for notwith- 

 standing the boat was completely empty, and considerable 

 exertions used, the progressive velocity did not exceed the rate 

 of two knots per hour. 



As a perpetual log, the spiral * has been used, and found 

 very useful in maritime surveying, by measuring a base line on 

 the water. This method of finding the distances of one head- 

 land from another, is rendered useless, if the spiral be placed 

 within a concave cylinder ; for the friction of the water against 

 the inside impedes its progress, consequently the distance shown 

 by the perpetual log is less than the true. Being requisite that 

 the number of revolutions made by the spiral be noted, it is most 

 advantageously done by a line and clock-work, one end being 

 attached to the log, the other to the clock ; but as the friction 

 of the clock-work, and the resistance the line meets with by 

 revolving in the water, impede the rotary motion of the spiral, 

 an allowance must be made for the error by placing the clock on 

 board a vessel towed in a stagnant canal, and measuring with a 

 perambulator, on the tract-path, the distance the vessel has 

 moved, the difference of the space shown by the perambulator 

 and the log is the error of the spiral ; should the water have a 

 slow motion, as is generally the case in our canals, the same 

 distance should be measured both with and against the stream, 

 and the mean of the two numbers of revolutions taken for finding 

 the error. 



The spiral may also be applied to measuring the velocity of the 

 wind ; and it is better in this case, as well as the former, to have 

 two leaves, or a double spiral, each leaf making a half revolu- 

 tion, than a single worm, which makes a complete turn. Mr. 

 William Gary, optician in the Strand, made me a machine of this 



* Invented by the celebrated Dr. Hooke. 



