Contents 



constant which is of greatest importance in terrestrial 

 physics 3 8 4 



When we have ascertained the supply of heat to the earth 

 we have to inquire what becomes of it. 



Fundamental principle : The heat which the whole earth 

 receives from the sun in the course of a year also leaves it in 

 the course of a year. 



The differential behaviour of the atmosphere to heat-rays 

 striking and leaving the earth's surface is an example of 

 KirchofPs law that a body absorbs by preference the rays 

 which it itself emits 385 



Wind and all mechanical atmospheric effects are due to 

 differences of density. These are produced not only by 

 thermal expansion of the air, but also by mixture with it of a 

 lighter gas ; such a gas is water-vapour 386 



Description of methods of measuring the thermal effect of 

 the sun's rays . 387 



Simultaneous observations made at the summit of Mont 

 Blanc and at Chamonix 390 



Observations on the Peak of Tenerife . . . .392 



Observations with the steam calorimeter in Egypt by the 

 writer and by Mr Michie Smith at a height of 7000 feet 

 above the sea at Kodaikanal in India. The highest rate of 

 distillation observed by Mr M. Smith with the same instru- 

 ment which I used in Egypt was 1754 cubic centimetres per 

 minute, whereas the highest realised in Egypt was i '501 cubic 

 centimetres. Therefore, at a height of 7000 feet above the 

 sea one-seventh more heat can be collected, measured and 

 used than was found possible on the banks of the Nile. But 

 it is the amount which arrives at the sea-level, or near to it, 

 that is alone of interest in the study of terrestrial physics . 393 



In a work entitled Strahlung und Temperatur der Sonne 

 (1899) J. Scheiner sums up the discussion of this subject by 

 giving 4 as the most probable value of the solar constant. 

 This statement is disputed and it is shown that values of the 

 solar constant of the order of 4 must be exaggerated. 



No comment on these remarks was offered, but the later 

 estimates of the solar constant began to fall. Attention is 

 called to them in a Postscript written in February 1911 . . 395 



Fery, who quotes 2-40 as the value of the solar constant 

 generally accepted in 1909, arrives by his own experiments at 

 the value 1 70, and he attributes the difference between these 

 two values to excessive correction for the absorption by the 

 atmosphere .396 



