OF GASES AT HIGH EXHAUSTIONS. 
395 
any great distance from the mercury in the pump ; and the tube packed with gold- 
leaf, which I formerly interposed between the pump and the apparatus, showed no 
trace of bleaching, and exerted no appreciable effect one way or the other on the 
results. The chief improvements in this instrument over Professor McLeod’s original 
design consist in the entire absence of joints ; the tubes and bulbs are all soldered 
together in one piece ; and the two air-traps, d d, which obviate the inconvenience at 
first encountered, of traces of air rising with the mercury and spoiling the vacuum in 
a. The globe a is also larger in proportion to the volume tube, and this tube is longer. 
These improvements are mere details, and in no way detract from the great beauty 
and merit of Professor McLeod’s valuable instrument. 
643. In the case of a body moving quickly through air of ordinary density the 
“work” is almost wholly represented by a wake of eddies left behind. The smaller 
the velocity of the body and the rarer the gas the less is the work spent in producing 
eddies compared with the work spent in directly overcoming friction. In these expe¬ 
riments the motion is so slow that even at full pressure the effect of eddies is well 
nigh or altogether invisible, and it is utterly insensible at even moderate exhaustions. 
The molar vis viva is rapidly converted by internal friction or viscosity into molecular 
vis viva (heat, thermometric—not radiant), and this is too rapidly dissipated to inter¬ 
fere with the observations. 
644. Before taking an observation the arm p (fig. 1) remains pressed against one of 
the stops. In this position the index line of light stands in the middle of the scale 
at 0, the divisions counting on each side from 0 upwards. The arm is then moved 
over to the other stop, and in a few seconds allowed to return to its original position 
by the action of the spring. This movement rotates the viscosity apparatus through 
a small angle, and sets the mica plate vibrating, the reflected line of light traversing 
from one side of the scale to the other in arcs of diminishing amplitude till it finally 
settles down once more at zero. The'amplitude depends on the manipulation, a slower 
turn being more effective in producing motion than a very quick one. Even if the 
arm p is turned at the same rate, in experiments with different degrees of exhaustion 
there will be no simple relation between the arc swung through and the viscosity. 
What is simply related to the viscosity is the logarithmic decrement of the arc of 
oscillation. The logarithmic decrement will of course involve the viscosity of the 
glass; but glass is so nearly perfectly elastic, and the fibre is so very thin, that this 
will be practically insensible except at the very highest exhaustions (652). 
The observer watching the moving index of light records the scale number reached 
at the extremity of each arc. The numbers being alternately on one and the other 
side of zero are added two by two together, to get the value of each oscillation. The 
logarithms of these values are then found, and their differences taken; the mean of 
these differences is the logarithmic decrement per swing of the arc of oscillation. 
The following illustration will render this plain. The observation is supposed to be 
taken with air at the pressure of 760 millims. :— 
3 F 
MDCCCLXXXI. 
