460 LIFE OF HORACE BENEDICT DE SAUSSURE 



cooled by radiation. He also reasoned from the diminution of 

 temperature by arithmetical progression as the height increased 

 that reduction of temperature would go on increasingly until it 

 reached the uniform condition of interplanetary space. Modern 

 researches on the upper air show that this is not the case, but that 

 the rate of diminution of temperature decreases as the height 

 increases, until at about 40,000 feet the fall of temperature 

 practically ceases and the stratosphere or portion of the atmosphere 

 above that level remains throughout at a uniform low temperature, 

 far higher however than the absolute zero which may be supposed 

 to prevail in interplanetary space. 



De Saussure took occasion to make a careful comparison 

 between the readings of a thermometer freely exposed to the 

 sun's rays and that of a similar instrument equally open to the 

 air but shaded from the sun. He found that the difference 

 between the readings was greatest in the morning and evening and 

 least at noon. This he believed to be due to the fact that wind 

 was usually strongest in the middle of the day, and the excess of 

 heat was then more rapidly carried off from the exposed bulb. 

 He rightly grasped the importance of using shade temperatures 

 as the true temperature of the air, and also of securing a strong 

 current of air across the thermometer bulb to ensure full contact. 

 For this purpose he devised the sling thermometer, the instru- 

 ment being suspended by a string which was whirled rapidly 

 through the air. 



Deluc had advised the use of temperature in the sun for 

 reducing barometric heights, and loosely explained the variations 

 in the readings of sun-thermometers as due to ' local causes.' 

 De Saussure, however, showed in a convincing way that it was 

 due to an inherent vice in the thermometer itself, the upper part 

 of the bulb being thicker than the lower and thus opposing a 

 greater resistance to heating when the rays fell at a high angle 

 than when they fell at a low angle. This, rather than the stronger 

 winds at noon, should probably also be held to account for 

 the difference in the diurnal variations in the sun and shade 

 thermometers. 



De Saussure devoted much time to observations on the electri- 

 fication and the transparency of the air, on the colour of the sky, 

 and on the rate of various chemical reactions at different levels. 

 Into these we cannot enter in detail, as they were all of minor 

 importance compared with his researches on evaporation and 

 atmospheric humidity. 



De Saussure 's name lives in Meteorology as the father of 

 Hygrometry. His Essai sur I'Hygrometrie, published in 1783, 



