PROFESSOR FORBES ON THE TEMPERATURE OF THE EARTH. 195 



2. In the Experimental Garden, adjoining the Royal Botanic Garden at In- 

 verleith, almost exactly 1 mUe NW. of the Observatory, and 280 feet lower, being 

 about 70 feet above the sea. The soil here is a remarkably pure sand, resembling 

 sea-sand, extending to a great depth, and including few pebbles of any size. The 

 precise locality was the flat summit of the rising ground, immediately to the 

 south of the large building or show-room in the Garden. The perforation of the 

 sand was exceedingly easy, owing to the uniformity and dryness of the strata. 

 The four thermometers were inserted in three boxes, near to one another, the two 

 shortest thermometers being placed together in the same hole. The surface is 

 garden mould, whereas, at the Observatory and Craigleith, it is covered with ve- 

 getation (in the former case interrupted by buildings and gravel walks). This 

 circumstance may not be without some effect. Sir John Leslie has observed* that 

 cold penetrates deeper through bare soil, or compact pavement, than through turf 



3. At Craigleith quarry, 2\ miles west from the Observatory, in a mass of 

 coal formation sandstone, which has for many years afforded an abundant and 

 durable building material for the city of Edinburgh. The spot chosen was situ- 

 ated in a field 50 yards north of the house called Craigleith Hill, immediately to 

 the east of the quarry, and about 75 yards distant from its north escarpment. 

 The field was under grass during the first two or three years of the observations, 

 afterwards under crop. The height above the sea is about 150 feet. The ther- 

 mometers were inserted here, as at the Observatory, in one hole, six or seven 

 inches in diameter at the top, and three at the bottom, which it required several 

 weeks to form with boring-u-ons in the usual manner. When the hole was 

 empty some water always flowed into it, and stood at a certain height, however 

 often removed. 



The insertion of the thermometers into the holes required the greatest pre- 

 caution ; the length and flexibiUty of the stems of the longest exposing them to 

 great risk of casualty. The operation was managed in the following manner. A 

 strong tripod, 12 or 15 feet high, was erected over the hole, and a ladder still 

 longer attached it, so that a man ascending it could command completely the 

 upper part of the instrument. The tube lay in the angle formed by two united 

 pieces of wood, similar to a roof gutter, where it was secured by loops of string. 

 Being raised, with this defence, into an erect position directly over the hole, the 

 loops were successively cut, and the thermometer allowed to slip from the wooden 

 shield, and to sink to the required depth. Dry sand was then poured in to half 

 the depth of the hole. The second thermometer was then similarly planted, and 

 so of the others ; the aperture being well closed round the tubes with clay puddle, f 



* Encyc. Brit., Article Climate. 



t Subsequently (May 1838), a quantity of Roman cement was employed to secure completely the 

 opening of the holes. 



