658 
ME. AIEY’S ACCOrXT OF THE COXSTEUCTIOX OF 
first order, but yet more widely separated than the breadth of the bar. Each lever of 
second order carries two rollers, upon which the standard-bar rests. 
y is the support of a lever of first order. 
z, z, the cross-ties of a lever of first order. 
a, a, the centres of motion of levers of second order. 
4, h, the cross-ties of a lever of second order. 
c, c, the rollers carried by a lever of second order, on which the standard-bar imme- 
diately rests. 
It will be seen that the pressures on the four different rollers c, c, c, c, canied by one 
le's^er-frame, are necessarily equal ; and that if the two lever-frames are planted symme- 
trically under the bar, the pressures on the eight different rollers are necessarily equal. 
Figure 13 contains a view and section, on an enlarged scale, of the ends of two end- 
bars in contact. The inclined lines show how the angles of the square bar are chamfered 
off so as to form an octagon ; which is turned at the extremity uito a cylmdiical fonn. 
Figure 14 represents the microscope commonly employed to read the graduations of 
the thermometers. The small pieces at the bottom of the frame embraced the edge of 
the flange of the drop-box, permitting the frame to slide longitudmally. The mode of 
illuminating the thermometer-tubes will be understood from this figm-e : fZ is a small 
hand-lamp, by which light was thrown horizontally into the reflecting prism e. By this 
reflexion, the light was thrown downwards upon the reflectmg prism j' ; and by the 
second reflexion the light was thrown horizontally upon the inclined card j", which 
served as a ground under the thermometer-tube j when 'viewed by the microscope 
from above. In order to facilitate the reading of the comb in the field of “siew, a card 
f was mounted, which, when the micrometer-wire had been brought to the end of the 
column of quicksilver or to the nearest division on the tube, was inclined to the posi- 
tion f, and received the light of the lamp, and fully illuminated the field of -^iew. 
This, perhaps, will be a proper place for inserting a memorandum by Mr. "W. Simms, 
.lun., on Mr. Sheepshanks’ practice in obser\dng the Standard Yards, with which ]\F’. W. 
Simms favoured me after the death of Mr. Sheepshanks : — 
“ The two bars to be compared were placed upon then supports Avithm the trough, 
at a distance of about 2 inches between their centres, and as nearly parallel as practi- 
cable, care being taken that each division should pass nearly mider the axis of the 
micrometer-microscope when the trough was Avheeled up to its place. 
“ There is a considerable amount of adjustment proAuded in the trough-carriage ; but 
Mr. Sheepshanks preferred having the apparatus so carefully arranged, that but little 
adjustment might be required at the time of comparison. 
“ I believe no observations were ever made on the same day on which the bars Avere 
arranged ; I frequently placed them for Mr. Sheepshanks in the evening, and he 
observed the following day ; so that generally eighteen homs Avere alloAved to intervene 
at the least. The trough was always covered up Avhen left, small shutters being placed 
over the holes in the covers through which the diAisions Avere seen. 
