EXPERIMENT STATION BULLETINS. 781 



while the minimum varied 'appreciabl3% but the difference Avas not as 

 great as was in the maximum in the preceding year, the greatest monthly 

 variation being only about 5V as against lU'F between the sand and 

 peat. 



Evidently, therefore, the cover of sand raised the maximum tempera- 

 ture, as well as it did the average, of the heavy types of soil and especially 

 of peat, to the same degree as that of the light classes of soil. And by 

 so doing it augmented their total intrinsic heat and their minimum tem- 

 perature did not fall as low as that of the light soils. As already stated 

 it accomplished this rise of temperature mainly by reducing the excess 

 of water-evaporation. 



The data of the fourth year during which the soils were not covered 

 with sand and their temperature was measured at 2, 4, and 6-inch depths 

 instead of 6, 12, and 18 as previously, are represented in Tables 63 to 

 74 inclusive. The results in these tables show that the magnitude and 

 order of amplitude in the 6-inch depth were practically the same as in 

 the second or corresponding year, when the soils remained uncovered, 

 but in the 2 and 4-inch depths, they were of course somewhat different. 

 The degree of fluctuation in these upper depths, and especially in the 2- 

 inch, was higher but surprisingly approximately equal in the various 

 soils. This means that the degree of amplitude in peat at the upper 

 depths, was almost as great as that in sand and gravel. 



The order and magnitude of the maximum and minimum at the 6-inch 

 depth in all the soils were also the same as in the second year, but in 

 the 2 and 4-inch depths they differed somewhat. The most important 

 difference, however, is that the magnitude of the minimum of the peat 

 at the 2-inch depth was equal to or slightly lower than that of sand or 

 gravel, while at the lower depths it was always higher. 



Temperature records taken at the upper i/4-inch depth by the mercury 

 thermometer with small bulb as already described, also show that the 

 peat at this depth attained in the morning, as a very general rule, a 

 slightly lower minimum than the sand and gravel, at the same depth. 



These unexpected large diurnal-nocturnal fluctuations and low mini- 

 mum temperature of the peat at the surface or upper depths as com- 

 pared with those of the light mineral soils, are undoubtedly due to the 

 poor heat conductivity of the material. On account of the low heat 

 transferring power of the peat, heat does not travel sufficiently rapidly 

 from the lower depths to replenish some of that which is being lost at the 

 surface and consequently it attains a low minimum, even though the 

 temperature at a slightly lower depth is very high. For the same rea- 

 son, during the insolation, the surface attains a high temperature. A 

 low minimum and high maximum therefore give a high fluctuation at 

 the surface. 



From the above it would seem therefore that the factor of poor heat 

 conductivity of peat predominates and exerts a slightly controlling in- 

 fluence on its temperature at or near its surface. 



t 



THE DEGREE) OP AMPLITUDE AT DIFFERENT DEPTHS AND THE LAW IT FOLLOWS. 



As already seen the magnitude of the diurnal-nocturnal amplitude 

 diminishes with depth. The remarkable fact, however, is that this de- 

 crease follows a mathematical law and indeed the geometric law, i. e.. 



