658 HISTORY OF SCIENCE. 



rain-gauges in use consist merely of a funnel of known area, and a 

 receiver of the collected water. The receiver is either itself graduated, 

 or the water is poured off and measured in a graduated vessel. Fig. 329 

 shows the rain-gauge of the Observatory of Paris. A metallic cpllecting 

 vessel A catches the rain, which a pipe conveys to the receiver B. It 

 is from time to time drawn at c into the graduated cylinder E. In 

 this way the rate of fall during a storm may be ascertained. The col- 

 lation of the rainfall statistics has led to many interesting results. Thus 

 a map of the rainfall of Great Britain shows at a glance that in the 

 hilly districts of the western part of our island the rainfall is more than 

 twice as great as in the eastern parts of the island. British meteoro- 

 logy is greatly indebted to the labours of Mr. G. J. SYMONS for the 

 collection and comparison of rainfall statistics. He, years ago, placed 

 himself in correspondence with a host of observers, mostly amateurs. 

 The number of regularly observed rain-gauges in this country amounted 

 in 1879, according to Mr, Symons, to about 2,200, by far the greater 

 number of the observers being volunteers. " Nearly all the observa- 

 tions are made at 9 a.m., local time ; and it often seems to me a 

 remarkable illustration of self-denial and willingness to help science, 

 that as 9 a.m sweeps across the British Isles from Lowestoft to the 

 West of Ireland, no matter how wild the weather may be, forth go some 

 1,500 or 2,000 persons of all social ranks, from peer to peasant, to make 

 their daily measurement of the amount of rain fallen." 



The force of the wind is measured either by its pressure on a surface 

 of known area, or by its velocity. The instruments for measuring 

 wind pressure are in general of a simple construction. A plate is ex- 

 posed perpendicularly to the wind, and the extent to which it is driven 

 back against the resistance of a spring measures the pressure. This 

 principle was first proposed by Bouguer. Another instrument consists 

 of a rectangular plate hung by a horizontal hinge at the top. The 

 angle which the plate makes with the vertical measures the force 

 of the wind. The plan of using the revolving vanes of a windmill 

 to measure the wind's velocity was first proposed by Wolff in 1743. 

 The instrument now almost exclusively used is that devised by 

 Robinson. It has four arms in the form of a cross, each bearing at 

 its extremity a hemispherical cup. The .theory of the instrument 

 shows that the cups will be moved by the wind with one-third of its 

 own velocity. 



