15 



strong yon are," which come on Blackheath every Easter Monday. 

 The first of these, which was made about the year 1746, by 

 M. Uouguer, in France, was simply a piece of canl-board, 6 inches 

 square, fixed perpendicularly across the end of a light rod which 

 pressed into a tube against a spring. The observer was to hold it 

 in his hand, present the face of the card to the wind, and notice 

 the jiressure as shown by the graduation of the rod. From this 

 very i)rimitive beginning has, in successive stages, been developed 

 the instrument now used here, at Greenwich, and elsewhere. This 

 was brouglit out liy Mr. Osier, of Birmingham, in 1836 ; the 

 principal improvement being the causing the pressure to act on a 

 wire which passes down the spindle and registers its motion below. 

 Instruments of this class are subject to a very serious objection, 

 which is that the ^vlnd, as it blows past the edge of the plate, 

 sucks away the air from behind, thus causing a partial vacuum, and 

 permitting the pressure on the face of the plate to show too great 

 an effect. No saiisfactory attempts have been made to get the 

 exact value of this error, wliicli has been variously estimated, from 

 something scarcely Avorth noticing up to nearly 100 per cent. : it 

 probably lies between these two extremes, but it is impossible to 

 say with any certainty what its amount really is. To do away 

 with this source of error, the late ^Ir. Charles Cator, of Beckenham, 

 adopted the device of filling in the back of the plate, which thus 

 became the base of a cone, but whether the instrument, so modified, 

 is indeed more trustworthy, has perhaps not been fuUy tried. 



A totally distinct type of instrument is that by which the 

 pressure is brought to act on a column of water, mercury, or other 

 liquid in a glass tube. The first and simplest form of this was 

 brought out by Dr. Lind in 1775, and consisted of a (J tube, 

 swinging freely on a vertical spindle, so as to form a direction 

 vane : the tube nearest the spindle is bent at right angles so as 

 to present its mouth to the wind, which, entering therein, brings 

 pressure on to the surface of some water in the tube, forcing it up 

 the other leg ; the difference of level in the two tubes gives a 

 measure of the force of the wind. The simplicity of this instru- 

 ment renders it a favourite, but it is subject to the disadvantage of 

 having an exceedingly contracted scale. To remedy this, various 

 attempts have been made. 



