EDMfi MARIOTTE 65 



which the law enables us to calculate. Mariotte divided the 

 whole atmosphere - the calculus had not yet been discovered 

 - into 4032 layers, each of which was assumed equal in pres- 

 sure throughout, and calculated the height of each single 

 layer. By summation the height corresponding to any pres- 

 sure could then be calculated. This is also to-day the basis 

 of all our knowledge of the distribution of pressure and 

 density in the atmosphere, and in particular, of the measure- 

 ment of height by means of the barometer. 



Another permanent achievement of Mariotte is the deter- 

 mination of the actual circulation of water upon the earth. 

 This circulation had hitherto been imagined as taking place 

 in the main subterraneously; the springs which were seen 

 to rise from the interior of the earth were imagined as fed 

 by subterranean water vapour, produced by the seepage of 

 sea-water, and perhaps also as rising from inexhaustible 

 subterranean stores of water. The quantity of rain water 

 was under-estimated, in so far as any attempt had been made 

 to arrive at its amount. Even Descartes held these views. 

 Mariotte, a scientist with Pythagoras' sense of the import- 

 ance of number, based his views on extensive measurements 

 of rainfall, which were made at his request and according to 

 his directions, and compared the average yearly quantity, 

 calculated as falling on the surface of the whole area of the 

 Seine springs, with the annual quantity of water flowing 

 down this river, which quantity he determined from the 

 velocity of drifting ships and the area of the river bed. It 

 appeared that the volume of rain was over six times as great 

 as was necessary to feed the river. From this, and from 

 numerous other observations which he made concerning the 

 seepage and reappearance of rain water in various areas, and 

 also concerning the variations in the water level of springs 

 and rivers at various times of the year, he drew the well 

 supported conclusion, that springs are fed by rain and snow, 

 which fall on the mountains; and that therefore the circulation 

 Fs 



