158 The Eev. T. R. Robinson's Description of an improved Anemometer. 



sisted of four light wdndmill-vanes, delicately mounted, the arbor of -which 

 had an endless screw, that recorded its revohitions by means of the elegant 

 arrangement now called the cotton-counter. This, the invention of wliich is 

 attributed by Willis to the late Dr. Wollaston (who, I beUeve, learned it from 

 Mr. Edgeworth), consists of two wheels of n and n + 1 teeth, driven by the 

 same screw ; a tooth of the first passes a fixed index for each revolution of the 

 vanes, and an index borne by it passes a tooth of the second for every n revo- 

 lutions.* In 1790, the hydrometric fly of Woltjlvn was proposed by its inventor 

 as an anemometer ; but Dr. Whewell is the first who appreciated in its full ex- 

 tent the importance of the space-measure, especially in its giving an integral in- 

 stead of a differential result. His memoir, published in the sixth volume of the 

 Cambridge Transactions, marks an era in the science, and, in my opinion, indi- 

 cates the only path of its progress. The instrument described by him has been 

 used by several observers, but most extensively by one whose energy and 

 talents were well adapted to establish its character, Sir William Snow 

 Harris. The results which he exhibited to the British Association in 1841, 

 while they fully proved the value of the principle, shewed, at the same time, 

 that the mechanical details were not sufiiciently perfect to carry out the views 

 of the inventor : in particular, the space traversed by the recording pencil is 

 (at least in moderate winds) not as the velocity, but rather as its square. 

 Harris proposed to investigate corrections for this, which, however, would be 

 different in each anemometer, and probably variable even in the same one. 

 This error arises from the small size of the vanes, which have, therefore, too 

 little power compared to the friction ; while that is greatly increased by the 

 same cause, as, from their great angular velocity, a complicated train of wheel- 

 work is required to bring down the speed of the recording point to a manage- 

 able amount. This report induced me to consider the subject carefully ; and 

 as it seemed possible to correct the defect in question, and some others which 

 I had observed in a similar instrument used by Captain Larcom, R. E., at 



in the sea. The wheels have seventy-two and seventy-three teeth, and the revolution of the second 

 wheel measures a mile. 



♦ A very convenient portable anemometer is made by furnishing a set of my hemispherical 

 vanes with such a counter. In that exhibited to the Academy, the radius of the circle described by 

 their centres = 5-6 inches, and the diameter of the hemispheres = 3-1 inches. If the number of re- 

 volutions which it makes in a minute be divided by 10, the quotient is the velocity in miles per hour. 



