348 The Realm of Nature 



the relative humidity of the atmosphere, the exact value of which for 

 each difference of temperature has been calculated and recorded in 

 tables by Glaisher. Dew-point hygrometers, in various forms, in- 

 vented by Regnault, Daniel, Dynes, and others, consist of a polished 

 surface, the temperature of which can be lowered by evaporating a 

 liquid, or by a current of iced water, until a film of moisture is con- 

 densed from the air. The temperature at which condensation takes 

 place is that of the dew-point, at which the vapour of the air becomes 

 saturated, and a table of the vapour-pressure of saturated vapour at 

 different temperatures gives the absolute humidity ( 158). 



442. Anemometers, or instruments for measuring the force of the 

 wind, are constructed either to record velocity or pressure. To 

 show velocity a series of hollow metal cups, mounted on a light 

 pivoted frame, are caused to revolve by the wind, and each revolu- 

 tion is registered by an arrangement like that of a gas-meter. 

 Experiment shows what ratio the speed of the revolving cups bears 

 to that of the wind. In pressure anemometers the wind blows 

 against a large flat surface, the pressure exerted on which is indi- 

 cated by the tension of spiral springs. These instruments, like all 

 others for measuring phenomena subject to constant variation, can 

 be made to write a continuous record on a revolving cylinder, from 

 which the exact direction, force, and velocity of the wind may be 

 ascertained at any moment. 



443. Deep-sea Soundings. The depth of calm water, when less 

 than 200 fathoms, can easily be found by letting down a lead 

 weighing 7 Ibs. by a line marked at regular intervals. The impact 

 of the lead on the bottom may usually be felt, and the line ceases to 

 run out, or at any rate, if too much line is let out, a sudden increase 

 in weight is felt when, on hauling it in, the lead is lifted off the 

 bottom. At great depths a very heavy sinker must be used : its 

 impact on the bottom cannot be felt, and the line runs out steadily. 

 In making a deep sounding, the line usually a fine steel wire is 

 marked at every 100 or 50 fathoms, and the intervals of time at 

 which each mark disappears in the water are carefully noted. On 

 account of the increasing resistance of the water on the lengthening 

 line the time interval lengthens gradually and uniformly ; but when 

 the sinker reaches the bottom there is an abrupt increase in the 

 time taken for the next 50 fathoms to run out, which is sufficient to 

 assure the officer in charge that bottom is reached. From depths of 

 3000 or 4000 fathoms no ordinary line or wire is strong enough to 

 haul up the heavy sinkers, which accordingly are so constructed as 



