168 CAPTAIN A. E. CLAEKE ON THE COMPAEISON OE ENGLISH AND EOEEIGN 
covers of the boxes : the microscopes used here are 12 inches in length, they are mounted 
on sliding plates so as to traverse the whole length of the thermometer-tube ; the verti- 
cally of their axes is also ensured. 
Method of Comparing . — Whenever practicable, the two bars which have to he com- 
pared are mounted side by side in the same box. Each bar is capable of being levelled 
(by raising or lowering the cradles or rollers on which it rests), or brought to focus under 
the microscopes. Each microscope has attached to it a level whereby the vertically, of 
the axis may be tested. It is usual to arrange a pair of bars for comparison on the 
afternoon of one day, and to commence observing the next day. The bars are visited 
three or four times each day ; a series of comparisons has generally consisted of about 
ten visits or comparisons ; and the bars are then dismounted, to be compared another 
time. All adjustments are frequently put out and renewed ; there is little use in mul- 
tiplying observations while none of the circumstances of the observations are changed. 
Consequently, as far as practicable, the comparisons of any two bars have been made in 
detached series ; thus the fear of constant error is diminished. It is generally assumed 
that the temperatures of two bars lying together in the box are the same, the minute 
differences which are sometimes found in comparing the readings of the thermometers 
being attributed to the thermometers themselves, which certainly do not always imme- 
diately indicate changes of two or three hundredths of a degree of temperature. The 
two bars are also made to interchange places, so that either one of them is next to the 
observer about as often as it is next to the piers. A temporary constant error may also 
creep in, if great care is not taken that the divided surfaces are clean ; minute particles 
of dust, almost indiscernible to the eye, or one such particle hanging about the edges of 
the line where it is to be bisected, will give a false result to all observations ; and it is 
of course undesirable to clean the surfaces, or even rub them gently, oftener than can 
possibly be helped. 
The observations made at any one visit to the bar-room are generally as follows : — 
(1) the two thermometers in each bar are read; (2) the bar A being adjusted to focus 
under the microscopes, three readings (bisections) of the microscope on the left are taken, 
and then three readings of the microscope on the right; (3) B being now adjusted 
under the microscopes, similar readings are made; (4) B is thrown out of focus by 
the levelling-screws, and being readjusted in focus under each microscope is observed 
as before; (5) A is observed a second time; (6) the thermometers are read again. A 
slight disturbance and rise of temperature is almost inevitably caused by the observer’s 
presence and the heat of the two candles. On the average, the second readings of the 
thermometers are 0 o, 035 above the first readings. 
In comparing any two bars, the greater part of the comparisons have been made at 
temperatures differing not more than 2° from the standard temperature of 62°, and a 
small portion at a temperature as low as possible ; thus the expansions of the bars are 
eliminated. 
With respect to personal error in bisections, this has been found to exist in the case 
