THE MASS OF A CUBIC INCH OF DISTILLED WATER. 
335 
When two standards or measures of extension are to be compared by means of the 
Whitworth comparator, first one standard and then the other is placed in a horizontal 
position between two contact points ; one of which is a fixed point, the other being a 
movable point or the termination of a micrometer-screw. Any difference, in parts of 
an inch, between the two standards can then be measured by means of the screw, the 
value in parts of an inch of one revolution or whole turn of the screw being known. In 
this comparator, the appreciation of the moment of contact is ascertained by means of a 
“ gravity-piece.” When the gravimeter was placed in position, one of its sides was in 
contact with the fixed point, its other side being in contact with the gravity-piece? 
which was interposed between the gravimeter and the screw, and the moment of 
contact between the gravimeter and the screw was then ascertained by gradually 
turning or releasing the screw until the gravity-piece fell by its own weight. 
In the general form of the ordinary micrometer-microscope comparator, or optical 
beam-compass, there are two fixed microscopes, the distance between which is deter¬ 
mined by comparison with a standard of length. The micrometer heads of the 
microscopes are divided into 100 divisions, the mean value of one division being equal 
to nearly 0'00003 inch; and the linear standards or measures of extension are 
compared by being alternately placed under the microscopes, a microscope being fixed 
over either end of the standard under observation. 
As the comparators were only required for the purpose of measuring small differ¬ 
ences between the standards and the gravimeters, it w r as unnecessary to verify the 
whole run of the micrometer screws ; but the particular parts of the screws used were 
verified by comparison with the subdivided inch D. 
There were no defining lines marked on the gravimeter Q by which its dimensions 
could be measured, although certain lines, as previously mentioned, were engraved on 
C and S, for the purpose of indicating generally the particular parts of the surface of 
the gravimeter brought under measurement. Therefore, in the microscopic compa¬ 
risons of Q, it became necessary to develop defining lines, visible through the 
microscope, and this was done after the method of Fizeau and Cornu. 
When the gravimeter was placed in position under the fixed microscopes, two 
pencil points, made of polished silver, were brought nearly into contact with the 
gravimeter, within O'OOl inch of the gravimeter at each end. The “points” did not 
therefore actually touch the sides of the gravimeter, but were so reflected as to form 
an apparent line of contact, visible through the microscope, on either side of the 
gravimeter. The actual distance between the two apparent lines formed by the 
“points” and their reflections, as measured by the fixed microscopes, v T as then com¬ 
pared with a standard of length which was placed under the microscopes after the 
removal of the gravimeter. 
Rates of Expansion of the Gravimeters by Heat .—The rates of expansion by heat of 
the three gravimeters were not ascertained by actual experiment, as the probable error 
which would arise in ascertaining the rate of expansion of a body having the particular 
