101 



that no undue excitement or perspiration take place, 

 as to the portion of the coil above the surface of the 

 pot. It may be urged by some that such treatment 

 is immaterial : let it be remembered that the average 

 amount of ground heat is well known to be several 

 degrees in advance of that of the atmosphere in most 

 parts of the globe. The average, however, does not 

 alone suffice to establish the fact that such a power 

 exists in nature ; what we want to know chiefly is the 

 greatest disparity — at what ratio it advances, and, 

 when at its height, the condition of vegetation. As 

 bearing on this point, we would make a quotation 

 from the United Gardeners' Journal of January 1 6, 

 1 847. The subject (p. 40) is the " Potato disease at 

 the Cape of Good Hope." The writer, Mr. R. 

 Smith, of The Oaks, South Africa, affirms, from ex- 

 perience, that the soil there, at the hottest period in 

 the year, frequently reaches 130 to 140 degs. at two 

 or three inches in depth. Mr. Smith does not state 

 the temperature of the atmosphere at that period — a 

 circumstance rather to be lamented. However, there 

 can be little doubt that the thermometer would 

 scarcely advance beyond 100 degs., and would per- 

 haps be somewhat between 90 and 1 00 degs. 



And here we would direct attention to the amount 

 of atmospheric moisture necessary. This, with re- 

 gard to the coilers, can scarcely be exceeded by ordi- 

 nary means. Let it, however, be remembered, in 



