Professor Moll on Mr Blanken's Fan-Gate Sluices. 95 



showing the value of the revolutions in miles, furlongs, and 

 • perches. 



When the roads are in good order, there will not be found 

 a difference of one revolution in five hundred, or not one in 

 the distance of a mile. 



It is curious, that, in wet weather, it always shows a small- 

 er measure than the true, from this reason — that the tire of 

 the wheel becomes lined with mud, which, in fact, adds to 

 the size of the wheel. When the mud is very stiff" and adhesive, 

 there is a difference of one in two hundred. Measuring with 

 the wheel of a carriage is more accurate than with the common 

 wheel odometer, the carriage wheel being larger, and the load 

 that it supports preventing the possibility of its slipping, and 

 also tending to prevent the jumping that takes place when an 

 unloaded wheel meets with any obstacle. The dial-plate of 

 this pedometer admits of travelling more than twenty miles 

 before the long hand makes a complete revolution, and the 

 distances can be seen by mere inspection without stopping 

 the gig. 



I tried this machine during five years of constant travelling, 

 while I was employed on the survey of Roscommon: It remain- 

 ed in good order, and I found it saved me time in supplying any 

 little omissions in the maps of my surveyors. 



This machine has also been used by a friend in London for 

 some years, and he has found it remain free from sensible wear. 



Edgewobthstown, March 25, 1825. 



Art. XVIII. — A Description of Fan-Gate Sluices, invent- 

 ed and Constructed by Mr J. Blanken, Jun. Counsellor 

 of State, Inspector-General of Public Works in the king- 

 dom of the Netherlands. By Dr G. Moll, Professor of 

 Natural Philosophy in the University of Utrecht. Commu- 

 nicated by the Author. 



In 1808, Mr Blanken published a paper on this invention, 

 which soon after was put to trial by order of government. 

 V it met with success, about twenty sluices, of large dimen- 



