Contents xxiii 



The representation of the faces of a crystal by great circle 

 planes, and that of the edges by diameters, all of which 

 necessarily meet in the centre, facilitates the choice of a 

 suitable system of crystallographic axes . . . .318 



A collateral advantage which the student gains by using 

 the globe in this way is the excellent mental discipline which 

 it affords. A very short experience developes enormously 

 the sense of direction. There is no operation in the geometry 

 of polyhedra or crystals which cannot, with the greatest ease, 

 be performed by the use of the globe 319 



Postscript containing examples of the use of the globe in 

 dealing graphically with the relations of the faces, edges, etc., 

 of a crystal 321 



It is impossible to illustrate examples without the aid of 

 the globe itself and the reader must have one before him in 

 order to make for himself the constructions described. One 

 metrosphere can be used with any number of globes, so that 

 separate details can be worked out on separate globes and 

 combined on others, thus avoiding the risk of mistakes due 

 to overcrowding 325 



No. 10. ON A SOLAR CALORIMETER USED IN EGYPT AT THE 

 TOTAL SOLAR ECLIPSE IN 1882. (From the Proceedings of 

 the Cambridge Philosophical Society, 1901, Vol. XI, Pt i, 



PP- 37-74-) 337 



While engaged in discussing questions connected with the 

 physics of the ocean, I found the want of definite knowledge 

 of the amount of solar heat which really reaches the surface 

 of the land or sea in a form which can be collected, measured 

 and utilised. There was no lack of actinometrical observa- 

 tions, but I found it impossible from them to obtain the data 

 that I sought. The aim of most observers has been to arrive, 

 by more or less direct means, at what is known as the solar 

 constant, that is, the quantity of heat which is received in 

 unit time by unit surface, when exposed perpendicularly to 

 the sun's rays outside of the limits of the earth's atmosphere. 

 For my purpose the amount of radiation arriving at the out- 

 side of the earth's atmosphere was of no importance. What 

 I wanted to know and to measure was the amount of solar 

 radiation which strikes the earth at the sea-level and is there 

 revealed as heat. It is the energy of this radiation which 

 maintains the terrestrial economy. Having the opportunity 

 of accompanying the expedition to Egypt for observing the 

 total eclipse of the sun on May i;th, 1882, I determined to 



