REMARKS ON SURFACE TEMPERATURE. 115 



and elevated station, do not differ materially from those at Bodmin, except 

 that the absolute minima are lower in the winter months. 



At the Spring Meeting for 1873, 1 presented and explained two diagrams, 

 since published in the last number of the Journal, which exhibited the 

 differences between the minima on grass at Penjerrick, near Falmouth, as 

 recorded by Mr. R. W. Fox, and those registered on the stand at the Obser- 

 vatory in that town. These differences are remarkable, reaching to 15 and 

 17 degrees ; and not less so are the low points (as e.g. 12 and 14 degrees of 

 frost), to which the surface temperature fell at Penjerrick, a spot where 

 delicate exotics thrive. The following statement of results of observations 

 at that place solely, will serve, though brief, for more strict comparison : — 

 1873. 1874 



. ^ ^ 



Dec. 28, Jan. 31. Feb. 7. March 12. April 6. May 9. 

 Therm. 6 in. above grass.. 24 26 26 18 26 26 



„ 3 feet „ ..25 27 27 20 27 27 



„ against N.E. wall . 86 37 38 31 38 87 



In my own garden the greatest differences between the minima during 

 the same season were as follows : — 



1873. 1874. 

 , A. , ^ — : /^ 



Nov. 7. Dec. 8. Jan. 3. Feb. 10. March 7. May 4, 

 Therm, on grass.. 35 35-3 33 38 21 27 



„ in shed . . 42 43-3 41 46 80 85 



The grass temperature here never fell below 21° in that winter; it was 

 24'5'' on March 12th, when it was 18o at Penjerrick. The season was mild. 

 In the present year the minimum here on February 25th was 13° on the grass, 

 29° in the shed ; and on March 21st 18o on grass, and 28o in the shed. Differ- 

 ences fully as great occur in clear nights in summer. 



The kind of surface on which the thermometer is placed makes a 

 material difference in the indications dependent on the extent of cooling of 

 that surface, which again varies with its radiating and conducting powers. 

 Thus, to give the relative cooling powers of a few substances as determined 

 by Mr. Glaisher, — long grass being 1000 : — 



Hare Skin is 1316 



Raw White Wool 1222 



Flax 1186 



Raw Silk 1107 



White Cotton Wool 1085 



Lamp-black Powder 961 



Glass 864 



Snow 657 



Garden Mould 472 



Sand 454 



Stone 390 



Gravel 288 



But it must be borne in mind that the result, especially as regards the 

 soils and surface of the earth, will vary with the seasons ; in other words 

 that the rate at which those soils are cooled at night will depend on the 

 extent to which they have been heated in the preceding day ; not to epeak 



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