GEODETICAL STANDARDS WITH THE ENGLISH STANDARD YARD. 
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planes, being slightly convex ; it is a bar of wrought iron two toises in length, and an 
inch and a half square in section, supported at one-fourth and three-fourths of its length. 
Standard Foot F is a bar 13 inches long by 1 inch square divided into inches. The 
extreme inches are further divided on inlaid strips of platinum, into tenths, and some of 
these tenths into hundredths. 
Thermometers . — The standard thermometers, to which all others have been referred, 
have had their errors determined by calibration measurement to every fifth degree, by 
means of an apparatus constructed for this purpose. The apparatus consists essentially 
of three parts: — (1) as a base, a heavy rectangular plate of metal 2 feet long lying hori- 
zontally, having at its extremities upright pieces 10 inches high ; (2) the top of these 
uprights are joined by a couple of steel rods parallel to one another and at the same 
height ; along these rods there slides a platform carrying a vertical microscope between 
the rods; (3) a sliding frame, supported immediately by the lower plate, moving in 
the direction of the length of the lower plate or of the steel rod above, carries the ther- 
mometer to be examined. This sliding frame is moved by means of a micrometer screw, 
which therefore draws the thermometer along backwards or forwards in the direction of 
its own axis. The bed of the thermometer is rendered truly horizontal, and the micro 
scope has a level attached by means of which its axis may be always kept strictly vertical. 
The thermometer is protected, as far as possible, from variations of temperature by being 
closely surrounded (except its upper surface) by metal ; and the detached column of 
mercury can be shifted from one position to another without touching the thermometer 
with the hand. The error of the mean length of a degree is determined by boiling the 
thermometer (in a horizontal position), and immediately after placing it in ice. 
The thermometers which record the temperatures of the bars are only 5 inches in 
length, each showing 20° range of temperature ; the degree is about a fifth of an inch 
in length, and subdivided to tenths. These thermometers are compared as often as 
necessary (and this is very often) with the standard thermometers. The apparatus for 
the comparison of thermometers consists of a water trough, 29 inches long by 9 inches 
broad and 9 inches deep (internal measurement), resting on three points, one of which 
is a levelling-screw. The thermometers rest on cross bars at the mid depth of the water, 
their tubes truly level. This trough stands on the ground, and the thermometers are 
read by means of a long microscope, which is mounted on a travelling platform, and of 
which the axis is made vertical. The degrees on the long standard thermometers are 
not generally subdivided ; and in order to read them accurately the following arrangement 
was adopted : on a small strip of plate glass are drawn a system of eleven equidistant and 
slightly converging lines ; this strip slides in the diaphragm of the microscope, and can 
be moved by the hand ; then, if it be required to subdivide a degree seen in the centre 
of the field of the microscope, the glass slide is moved until the outer lines of the system 
coincide with the bounding lines of the degree. Thus tenths are immediately read, and 
the hundredths can be estimated. 
In order to read the thermometers when lying in the bars, orifices are provided in the 
