656 
ME. AIET’S ACCOUXT OF THE COXSTEUCTIOX OF 
Figure 4 is a plan of the water-trough, as mounted on the platform, and carrying the 
drop-box, in which the two Standards to be compared are supposed to be placed. 
The upper surface V of the water-trough is cast in a pannelled form. 
Y is the drop-box, made of brass, resting by its flanges all round upon the upper 
surface of V. 
Z is the cover of the drop-box. 
a, a are two handles of the cover. 
b, b are two long slits, for observing the stalks of two thermometers enclosed in the 
drop-box. 
b', b', two slits through which light is thrown from the upper reflecting prism to the 
lower reflecting prism, to illuminate the thermometer-scales. 
c, c, c, c are four holes, through which the deflning points of two Standards are 
viewed by the microscopes G. 
Figure 6 is a longitudinal section of the water-trough, the drop-box, and the quick- 
silver-box, showing one of the bars as partially floating in the quicksilver, for obseiwation 
with the microscopes. (The bars intended to be placed in quicksilver were all covered 
with gold-beater’s skin.) 
d, d are two blocks supporting the quicksilver-box. 
e is the quicksilver-box of cast iron. 
f is a swinging stirrup, on which the middle of the bar rests very slightly. 
g, g, adjusting screws which act on the immediate support of f. 
h, the Standard Bar under observation, floating almost freely in the quicksilver. 
i, i, the well-holes in the bar, sunk to the depth of half its thickness ; in the bottom 
of each well-hole is flxed a gold pin, on the surface of which the deflning mark is traced. 
In all the bars ultimately constructed by Mr. Sheepshanks, the defining mark is a fine 
line, transversal to the bar ; accompanied by two lines parallel to it, one on each side at 
the distance O’Ol inch; which were intended to give the means of ascertaining the 
value of the divisions of the micrometer. There are also two longitudinal lines ; and 
the point of the transversal line intended to define the Standard Measure is that which 
is midway between the longitudinal lines. 
j, j, two bent thermometers. It is to be remarked that there is a longitudinal par- 
tition in the quicksilver-box, and that two large and deep notches are cut in this pai'- 
tition, in which the bulbs of the thermometers are lodged, and through which the quick- 
silver flows. The stalks of the thermometers rest on the edge of the partition. (In a 
few bars, holes were sunk in the bars themselves, and bent thermometers with small 
bulbs were placed in these holes, in quicksilver, I believe). 
j', j', two long rectangular glass prisms, for turning a beam of light, thro\\m vertically 
through the slits b', b', into a horizontal direction, by internal total reflexion, in order 
to illuminate the thermometers jj . 
j", j", two paper cards in an inclined position, beyond and below the thermometers, 
upon which was thrown the beam of light passing horizontally from j', j', and which 
formed a white ground for the vertical vieAV of the thermometer-tubes. 
