Fig. 38 shows an original apparatus which I designed and made for my own use; with ordinary anemometers it is necessary to count the number of turns per minute in order to ascertain the velocity of the wind. I wanted something that would indicate the velocity and the direction of the wind without any figures or formulæ. I therefore made the apparatus shown in the drawing, in which _a_, _a_, is a metallic disc 13·54 inches in diameter, giving it an area of exactly 1 square foot. This is attached to the horizontal bar _b_, and the whole mounted on two bell crank levers as shown. When the wind is not blowing, the long arms of these two levers assume a vertical position, and the spiral spring _h_, is in exact line with the pivots on which these levers are mounted, and has no effect except to hold the levers in a vertical position. As the spring has very little tension in this position, and as it requires a considerable movement in order to give it tension, the arms _c_, _c_, and the bar _b_, _b_, are very easily pushed backwards, but as the distance through which they travel increases, the angle of the lever changes and the tension of the spring increases at the same time, so that when the disc is pushed backwards to any considerable distance, a strong resistance is encountered. Had I made this apparatus so that the pressure acted directly on the spiral spring, the spaces on the index indicating low velocities would have been very near together, while those indicating high velocities would have been widely separated, but with this device properly designed, the spacing on the index became regular and even. The index being very large enabled one to read it at a considerable distance, and at the same time, it acted as a tail and kept the apparatus face to the wind. The spaces of the dial were not laid off with a pair of dividers, but each particular division was marked by an actual pull on the bar _b_, through the agency of a cord and easily running pulley and weight. The markings, however, were not correct, because Haswell’s formula was employed in which the pressure of the wind against the normal plane is considerably greater than with the more recent formula, which is now known to be correct. Haswell’s formula was V² × ·005 = P, and the recent formula P = 0·003 × V², where P = pressure in lbs. per square foot and V = velocity in miles per hour. In my experiments, I also employed a very well made and delicate anemometer by Negretti & Zambra. [Illustration: Fig. 39.--Apparatus for testing the lifting effect of aeroplanes at a low angle and extremely high velocity. _a_, _a_, the aeroplane; _b_, lead weight; _c_, long and slender pine rod; _d_, tail for keeping the apparatus head on and ensuring its travelling straight through the air; _e_, the point of suspension, also the centre of gravity. When this apparatus was travelling at the rate of 80 miles an hour, it gave a lifting effect of about 36 lbs., which is about 7 lbs. per square foot.]