Vanes and Anemometer. 



lix 



ing of tlie Observatory. This vane indicated tlie direction of the lightest winds, 

 and the direction of the wind was generally taken from it after November 13, 1844. 

 On December 4, 1846, a light frame covered with oiled silk was substituted for the 

 turkey^ feathers. The direction of the wind is indicated in this volume by the num- 

 ber of the point of the compass, reckoning N = 0, E = 8, S = 16, W = 24. 



101. The anemometer, the invention of Mr R. A die, of Liverpool, was made by 

 Messrs Adie and Son, of Edinburgh; it occupies the north-east corner of the Obser- 

 vatory. This instrument will be best under- 

 stood by a reference to the annexed figure : 

 a is a cistern containing water to the level 6, 

 c being a turn-cock for letting the water off 

 to the exact level, and d a glass-gauge to shew 

 when the water becomes too low, from eva- 

 poration or otherwise ; an inverted vessel e 

 is suspended in the water by a cord passing 

 over the wheel/, whose axle rests on friction- 

 rollers at g and h; « is a spiral, which has a 

 cord wrapped on it carrying a weight A;, which 

 balances the vessel e ; Z is a dial, graduated 

 on the face near the circumference ; m an in- 

 dex, attached to the common axle of the 

 wheel and spiral ; n a loose index under the 

 index w, which the latter carries forward by 

 means of a projecting pin near the extre- 

 mity ; a tube passing under the cistern a, 

 which, entering the bottom, proceeds upwards 

 within the vessel e till its open extremity is 

 above the level of the water in a neck of the 

 vessel e ; the other end of the tube o is six 

 feet above the outer wall of the Observatory, 

 where it is capped by a vane p ; at the top 

 of the tube o three brass rods are joined, 

 which carry a small tube in which a pin within 

 the top piece q rests or turns ; the tube o is 

 double at the top, containing between the 

 tubes a quantity of mercury to the level r, 

 the continuation of the cylindrical body of 

 the vane enters the mercury, and a double portion s acts as an outer cover to the mer- 

 cury cistern ; « is an aperture, 2 inches square. When the wind blows, this aperture is 

 presented to it, the wind then presses on the column of air within the tube o (being 

 prevented from escaping under the vane by the mercury), and ultimately on the top 



