Dr. Robinson entertained the theory that the cups (measuring from their centres) revolved with one-third of the wind’s velocity; and this theory having been fully supported by experiment, due allowance has been made in graduating the wheels so that the true velocity is obtained by direct observation. In an improved form of this anemometer the hemispherical cups are retained, but the index portion of the instrument consists of two graduated concentric circles, the inner one representing five miles divided into 10ths, and the outer one bearing 100 divisions, each of which is equivalent to five miles. At the top of the dial is a fixed index, which, as the toothed wheel revolves, marks on the inner circle the miles (up to five) and 10ths of miles the wind has travelled, while a movable index, which revolves with the wheel, indicates on the outer circle the passage of every five miles. [Illustration: 58. Improved Anemometer. Scale about 1/5.] This instrument can be made very portable by removing the arms bearing the cups, when the whole may be packed with iron shaft in a case 15 × 13 × 4 inches. It may be placed in any desired position by screwing the iron shaft supplied with it into the hole provided for the purpose, and fixing the apparatus on a pole or on an elevated stand, if possible, in an open space exposed to the _direct_ action of the wind. If, when placing the instrument, the hands stand at 0, the next reading will, of course, show the number of miles the wind has traversed; but, should they stand otherwise, the reading may be noted and deducted from the second reading, thus: Suppose the fixed index points to 2·5 and the movable index to 125, the reading after 12 hours may be 200 on the outer circle and 3·0 on the inner circle: these added together yield 203. By deducting the previous reading 127·5, we have the true reading—viz., 75·5 miles as the distance travelled by the wind. Having obtained the velocity of the wind in this manner in miles per hour, the table on page 83, from Col. Sir Henry James’s “Instructions for Taking Meteorological Observations,” will enable the observer to calculate the pressure in pounds per square foot. WEATHER NOTATION. The following letters are used to denote the state of the weather:— _b_ denotes blue sky, whether with clear or slightly hazy atmosphere. _c_ „ cloudy, that is detached opening clouds. _d_ „ drizzling rain. _f_ „ fog. _h_ „ hail. _l_ „ lightning. _m_ „ misty, or hazy so as to interrupt the view. _o_ „ overcast, gloomy, dull. _p_ „ passing showers. _q_ „ squally. _r_ „ rain. _s_ „ snow. _t_ „ thunder. _u_ „ ugly, threatening appearance of sky. _v_ „ unusual visibility of distant objects. _w_ „ wet, that is dew.