The Rev. T. R. Robinson's Description of mi Improved Anemometer. 159 



Mountjoy Barracks, I obtained permission from the governors of the Armagh 

 Observatory (who had already directed me to erect an anemometer) to carry 

 into effect my views. After some preUminary experiments, I constructed in 

 1843 the essential parts of the machine, a description of which I now submit 

 to the Academy, and I added in subsequent years such improvements as were 

 indicated by experience. It was complete in 184(J, when I described it to the 

 British Association at Southampton ; so that I have had sufficient opportunity 

 to ascertain its efficacy. 



In contriving it, I was guided by the following principles : 



1. The moving power should be so great in comparison of the friction, that 

 the correction due to the latter may be inconsiderable. It should also be easily 

 applied. 



2. One means of effecting this is to have surfaces which receive the wind"s 

 impulse as far from their axis of motion as is consistent with strength. This 

 satisfies the second condition, namely, that they shall be acted on by a large 

 section of the ciurent, and thus give an average result. When the vanes are * 

 as small as those used by Whewell, they may give measures far different from 

 the general velocity, if met by those partial streams to which I have referred. 



3. The movement of those surfaces should be as slow, relatively to that of 

 the wind, as may be consistent with a sufficiency of moving power: this lessens 

 the train required to bring down the speed of the recording point, and also 

 diminishes the wear and tear of the whole machine. 



4. It seems desirable that it should act without requiring any special pro- 

 vision for turning it in the direction of the wind. 



5. The structure should be such, that all made after the same type will 

 give identical results. 



The third, fourth, and probably the fifth of these conditions, are against the 

 vertical windmill as a measure. Its vanes never move slower than the wind, often 

 three or four times as fast at their outer extremities.* With the best guiding ap- 



* This must generate a considerable centrifugal motion in the air dragged round with the 

 vanes, which wOl complicate the direct impulse of the wind. Its effect in large windmUls is illus- 

 trated by a remarkable fact, observed in Holland by the late Mr. Nimmo, that in some of the best 

 of them, the weathering at the extremity of the saU is ,mjative. This can only act by preventing 

 the escape of the air. An effect of this kind must be difficult to calculate. 



