120 Proceedings of the Royal Society of Edinburgh. [Sess. 
X. — The Cooling of the Soil at Night, with Special Reference 
to late Spring Frosts. By Captain T. Bedford Franklin, 
B.A. (Cantab.). Communicated by The General Secretary. 
(MS. received April 7, 1919. Read May 5, 1919.) 
SUMMARY. 
SECTION PAGE 
I. Introduction . . . . . . .120 
II. The Physical Constants of the Soil considered, and the Notation 
employed . . . . . . . . .123 
III. The Method of taking Observations — 
(a) Radiation and Relative Humidity .125 
( b ) Latent Heat of Freezing . . . .125 
(c) Upward Conduction .... 126 
( d ) Cooling of the Soil ...... 1-27 
IV. Available Data and Conclusions ...... 128 
Y. The Thermal Effect of a Layer of Poor Conducting Material on 
the Soil ......... 131 
YI. The Effect of Re-aeration of the Sjoil after Heavy Rain . . 133 
VII. The Thermal Effect of screening the Soil from Radiation, 
Evaporation, and CoLd Precipitation ..... 133 
VIII. Conclusions ......... 136 
I. Introduction. 
In this paper I am mainly concerned with the surface soil temperature, 
but as we are accustomed by long usage to think of the grass minimum 
temperature as determining the occurrence or non-occurrence of frost, a 
few words of explanation on this point is perhaps necessary at the outset. 
The grass minimum on nights of rapid radiation certainly does fall 
considerably below the surface soil minimum, but this is due to the 
very fact that it is the grass minimum, i.e. the air temperature just on 
the grass. 
Now grass is a notoriously bad conductor, and Dr Aitken, in his paper 
“ On Dew,” * mentions an instance when the grass minimum was 18*5° F. lower 
than the temperature of the surface of the soil beneath it. I myself, on 
February 10, 1919, observed a grass minimum of 15° F. when the surface 
of the soil beneath was 33° F., and a single primrose was flowering with 
* “On Dew,” by John Aitken, LL.D., F.R.S., Transactions of the Royal Society of 
Edinburgh, vol. xxxiii. 
