762 PROF. W. H. MILLER ON THE CONSTRUCTION OF THE NEW STANDARD POUND. 
comparison. Mr. Vandome’s pound, Mr. Bingley’s troy pounds, and the Bank of 
England troy pound, were all constructed, along with the lost standard, in 1758 by 
Mr. Harris, Assay Master of the Mint. These were referred to, at the suggestion 
of Professor Schumacher, in the hope of 'arriving at a knowledge of the volume of 
the lost standard, which unfortunately had never been determined by weighing it in 
water. For, as long as the volume of the lost standard remains unknown, the weight 
of the air displaced by it, and, consequently, its absolute weight, is uncertain within 
limits far exceeding the errors of weighing. 
The first step in the process of arriving at the weight of the lost standard, was 
obviously to compare among themselves the different troy pounds with which 
the lost standard had been compared by Captain Kater and Captain v. Nehus. 
These comparisons were made with a balance of extreme delicacy procured from 
Mr. Barrow. In its construction it nearly resembles the balances of the late 
Mr. T. C. Robinson. The beam is made sufficiently strong to carry a kilogramme in 
each pan. The middle knife-edge is about r93 inch long, and rests, when the balance 
is in action, throughout its whole length on a single plane surface of quartz. The 
surfaces of quartz which rest upon the extreme knife-edges, and from which the pans 
are suspended, are also plane. The distance between the extreme knife-edges is 
about 15'06 inches, the length of each about 1'05 inch. 
Instead of having an index pointing downwards, as is usual in balances of this 
description, the beam has a pointer at each end, and a graduated scale is carried by 
an arm attached to the pillar of the balance at a little distance behind the left-hand 
pointer. Affixed to the right-hand end of the beam is a thin slip of ivory, a little more 
than half an inch long, divided into spaces of about O'Ol inch each, or subtending an 
angle of about 5' each at the middle knife-edge. This scale is viewed through a com- 
pound microscope, having a single horizontal wire in the focus of the eye-piece. The 
distance between two divisions of the scale, as seen through the microscope, subtends 
an. angle of about 37'. This contrivance for determining the position of the beam at 
the extremity of an oscillation, was found so superior to a scale and pointer viewed 
with the naked eye, that after a trial of a few days, the scale at the left hand was 
found to be a useless incumbrance and was accordingly removed. A screen was 
interposed between the observer and the front of the balance case, having a small 
opening opposite to the eye-piece of the microscope, through which the scale could 
be seen. 
In order to admit of the employment of a large vessel of water in observations for 
finding specific gravities, the base of the balance has an opening immediately under 
the right-hand pan, capable of being closed when not in use by a sliding plate of 
brass. A corresponding opening exists in the table on which the balance stands. 
The vessel of water is placed under the table, and the wire by which the object to be 
weighed in water is suspended from a hook under the right-hand pan, passes through 
the openings in the base of the balance under the table. 
