154 DRY-FARMING 
In 1868, Nessler found that during six weeks of an 
ordinary German summer a stirred soil lost 510 
grams of water per square foot, while the adjoining 
compacted soil lost 1680 grams, —a saving due to 
cultivation of nearly 60 per cent. Wagner, testing 
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Fic. 35. Tillage forms a loose dry mulch on the land surface, which 
prevents evaporation. 
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the correctness. of Nessler’s work, found, in 1874, 
that cultivation reduced the evaporation a little more 
than 60 per cent; Johnson, in 1878, confirmed the 
truth of the principle on American soils, and Levi 
Stockbridge, working about the same time, also on 
American soils, found that cultivation diminished 
evaporation on a clay soil about 23 per cent, on a 
sandy loam 55 per cent, and on a heavy loam nearly 
13 per cent. All the early work done on this subject 
was done under humid conditions, and it is only in 
