132 _ Further experiments on the (Marcu, 
Everest to submit the bar to a new inquiry, attended with every 
precaution to insure confidence. 
The process adopted was framed on the principle pursued on the 
former occasion, namely the employment of a steam-pipe, to heat the 
metal uniformly to the boiling point. The section of the bar, 25 inches 
by 3, prevented its application in the same simple manner, by insertion 
in a leaden pipe; and it was determined to employ micrometers on 
the microscope principle of Troughton to read off the expansions ;— 
a new apparatus was therefore constructed by Mr. H. Barrow, H. C. 
Mathematical Instrument-maker, of which the following description, with 
reference to the perspective view in Plate VII. will explain the nature. 
A double cylindrical case ab (fig. 3) was made, 9 feet 11 inches in 
length, and four inches in outside diameter, the inner cylinder being of 
copper, the outer case of tin. The space between them was shut in at the 
two ends, with perforated discs, so as to allow the bar to be inserted 
freely into the inner tube. The bar was supported in the tube upon 
two brass rollers, enclosed in the steam-tight square boxes at c, d, and 
situated at the same distance apart as the rollers upon which the bar is 
always supported in its own wooden case. (fig. 1.) 
The tubes were pierced through from above in four points e, f, g, 
h, for the introduction of thermometers, the bulbs of two of which 
(f, g,) penetrated into deep cavities apparently provided for the pur- 
pose in the bar itself ; these were filled with mercury, to insure the right 
reading of the temperature of the bar. The cylinder, a 6, was supported 
on two of the brass tripods of the measuring apparatus, technically 
called camels, k, 1, which are provided with vertical and horizontal 
screw motions to adjust the position of the bar. The steam was admitted 
from my small engine bya pipe at the northern extremity 4, and suffered 
to escape freely from the waste pipe m at the other end. 
Two micrometer-microscopes, , 0, were firmly attached by screws to 
two isolated solid blocks of stone, p, q, built upon the stone pavement of 
the laboratory at the requisite distance apart; the focus of the object 
glasses being adjusted in true verticals to distinct vision of the minute 
dots on the silver discs of the standard bar, when the latter was itself 
adjusted horizontally to a perfect level by means of a theodolite placed 
on the opposite side of the room. 
The object of the double cylinder, according to the original design, 
was, to encompass the bar with a steam jacket, and thus heat it to the 
requisite point without allowing the steam itself to come in contact with, 
and thus to corrode, the iron ; as well as to prevent its escape from the 
two open ends, which would incommode the glasses of the microscopes : 
