TEMPERATURE OF THE EARTH IN THE SOCIETY'S GARDEN. 99 



XVI. — Observations made with reference to the Temperature of 

 the Earth iti the Garden of the Horticultural Society at 

 Chiswick. By Robert Thompson. 



Whilst attention has been directed for many years to the tem- 

 perature of the air, that of the earth has been comparatively but 

 little noticed, either at home or abroad. That it is necessary to 

 understand the one as well as the other will, I presume, be rea- 

 dily admitted ; for the fitness of soils to produce certain crops 

 frequently depends on the condition of the subsoil as regards 

 temperature. What constitutes a good condition or a bad con- 

 dition in this respect cannot be exactly known until determined 

 by thermometrical experiments. In a favourable soil and cli- 

 mate, vegetation may be seen thriving well ; in apparently 

 similar circumstances it often thrives badly : tlie cause is gene- 

 rally found to be owing to a cold subsoil. This on a large scale 

 cannot be heated artificially ; but in many cases that which 

 occasioned the coldness can be removed. When the temperature 

 of the soil does not rise in due proportion with that of the air in 

 summer, it is generally owing to the presence of spring w ater ; 

 hence the vast improvement consequent on its removal by drain- 

 age. This is one mode by wliich the temperature of the soil 

 can be elevated in the growing season, an etfect which is highly 

 deserving of particular investigation by means of ground ther- 

 mometers. 



The following abstract is drawn up from observations made 

 daily in the garden of the Society, with apparatus wliich it will 

 be proper in the first place to describe. 



Two thermometers were constructed by Mr. Newman, the 

 tube of the one being a foot in length below the commencement 

 of tlie scale, and the other two feet. Both extended a foot above 

 ground when their bulbs were respectively one foot and two feet 

 below the surface. The stems or tubes of the thermometers 

 were enclosed as far as they extended below ground in copper 

 tubes about f inch in diameter. To the tops of these the scale 

 was joined by copper straps, continued from the tubes, and along 

 the back of the piece of boxwood on which the scales corre- 

 sponding to each thermometer were engraved ; the copper form- 

 ing the protecting tubes nowhere touched the thermometers. 

 The cavity between the stem and tube was filled up with finely- 

 powdered charcoal, rendering it as compact as possible by pour- 

 ing in water occasionally during the process of filling : an incli 

 at top was filled in with clayey loam. The lower extremities of 

 the copper tubes terminated in a point, a little above which, 

 opposite to the bulbs, openings were cut in order that the soil 

 might get in contact with the bulbs. Holes were made in the 



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