Of these, a very ingenious one was made by Dr. "Wollaston 

 early in the present century. A (J tube c has its legs leading into 

 A and B the two divisions of a box, of which a is air-tight, except 

 as regards the opening d, whilst b is lightly covered. (Fig. 3.) 



The lower part of the tulie c contains water to the level e f, but 

 the upper part of each leg contains oil, which reaches for some way 

 into each of the divisions a and b. When the opening d is 

 presented to the wind, the water is depressed at E and raised 

 at f; but the head of water at f is partially balanced by the 

 oil above e, so that it indicates a pressure corresponding, not to 

 its own specific weight, but to the excess of its weight above that 

 of the oil. In this way the scale was magnified some 10 times, but 

 the simplicity of the original instrument was destroyed ; and instru- 

 ments of this type have not been received with any great favour. 



All together, the difficulties in the way of satisfactorily 

 measuring the pressure of the wind on a flat surface are so great 

 that in this country they have been virtually given up. At the 

 present moment there are probably not more than a dozen " Osier's 

 Anemometers" actually fitted; certainly there are not more than 

 twenty. What has been almost universiUy accepted is the en- 

 deavour to measure the velocity of the wind, and from the velocity 

 to calculate the pressure. The leading feature of all velocity 

 anemometers is a fan, which is turned round by the wind ; the 

 fan may be of different forms, resembling the sails of a windmill, 

 or the paddle-wheel of a steamer ; but that now adopted by general 

 consent not only in England but all over Europe and America, is 



