
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 mile 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 Joun Lesiie has observed* that 
cold penetrates deeper through bare soil, or compact pavement, than through turf. 
3. At Craigleith quarry, 21 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-irons 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 flexibility of the stems of the longest exposing them to 
ereat 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.t+ 
* Encye. Brit., Article Curate. 
{ Subsequently (May 1838), a quantity of Roman cement was employed to secure completely the 
opening of the holes. 
