
PROFESSOR FORBES ON THE TEMPERATURE OF THE EARTH. 193 
whole was entrusted to Mr Apig, to whose experience and skill I am greatly 
indebted for the completion of the whole without any material accident. 
For the larger thermometers it is of great consequence that as little liquid 
as possible should be contained in the stem of the instruments, otherwise the 
apparent expansion of the column will depend greatly upon the variable tem- 
perature of the different parts of the stem, as well as on that of the level. This 
correction being difficult to apply with mathematical exactness, it was desirable 
to make it as small as possible (although it would be unwise to overlook the 
correction altogether, as most observers have done). I accordingly had twelve 
tubes drawn, each of about 26 feet in length; of the external thickness nearly of 
a common barometer tube (about half-an-inch), but whose internal diameter 
was nearly capillary. These were carefully examined throughout, by means of a 
column of mercury passed through them. 
The proportional numbers, representing the calibre of the tubes, were en- 
tered in a table now before me, corresponding to every foot of their length; and 
the tubes were numbered, so that the degree of uniformity of any portion could 
at any time be ascertained. From these tubes twelve lengths were cut from the 
most uniform parts, amounting altogether to about 144 feet, for the construction 
of the three sets of thermometers. 
It is to be understood that the capillary tube now spoken of was made with 
a view to reach the surface of the ground, above which the tube should expand 
into one having degrees of a convenient length. So small, indeed, was the stem 
compared to the bulb, that a degree of Fahrenheit in the capillary tube would 
have occupied, in one case, a space not less than 51 inches long. The wide tube 
to which the scale was attached, had a bore of about +5 inch, and was made long 
enough to include the expected range of temperature at their respective depths. 
These ranges were, however, in some cases rather under-rated. 
The bulbs were cylindrical, and varied in size from about 6 to 8 inches long, 
and 14 or 25 wide. They were blown at the glass-house separately from the 
tubes. The deepest thermometers had the largest bulbs and longest degrees, be- 
cause the required range was less. 
From the length and fineness of the tubes much trouble would have been ~ 
experienced in filling the thermometers in the usual way. The lower end of the 
cylindric bulb was, therefore, drawn out into a tube, by which the liquid (freshly 
boiled alcohol, slightly coloured) was admitted, and it was drawn in by the action 
of a syringe fixed at the extremity of the long stem. Both ends were then closed 
_ in the usual way, an expansion being left at the top as in common alcohol ther- 
| mometers, but most necessary in this case, in order to allow for the changes of 
temperature to which the instruments were exposed before sinking them in the 
ground. 
The graduation was one of the most delicate parts of the construction. The 
VOL. XVI. PART II. 3 C 
