50 
Pr.OFESSOR E. G. COKEE AND MR. S. B. CLEMENT OX THE VARIATIOX 
found to answer all requirements. Errors due to the inequalities of the tube 
^y 0 Pe uot ]‘id of hv measurements taken on both tubes, while a suitable correction 
was made for temperature. At the low velocities, considerable difficulties were 
experienced. Tlie difference of heads between the two sections at the lowest 
velocities was about ’005 inch of mercury, and as this must be read very accurately, 
it became a matter of such difficulty that a new gauge was made and filled with 
carbon bisulphide. A number of trials were made, but it was found that the carbon 
bisulphide was very sluggish in action, and, unless a very considerable time was 
allowed between every two successive runs, its readings could not be relied upon. 
Another and more serious defect was the shape of the falling meniscus, which rarely 
assumed its proper form, so as to afford a definite measurement. This was due to 
the adherence of the carbon bisulphide to the glass; and in spite of repeated 
cleanings with different re-agents, no decided improvement was made and the gauge 
was abandoned. A return was made to the mercury gauge, and the cathetometer 
yms replaced by micrometer microscopes, vdiich had been carefully calibrated before¬ 
hand. These aflhrded much better results, but the observations were still irregular. 
Finally, tlie solution of the difficulty was found by turning the U-gauge upside 
down and imprisoning in its upper part a fixed column of air above the water in 
both limbs of the gauge. 
At first sight this might not seem to be a good arrangement, since any small 
variation of temperature vail affect the imprisoned volume of air considerably, hut 
this affects both legs equally, and there is no error from this cause. A possible 
source of error is the creeping of air from the pipe to tlie gauges. This is 
extremely unlikely, as the air in this case must first descend. If any liberation 
of air occurred from the water, its effect in altering the gauge would only be 
momentary. 
In practice, this gauge proved extremely sensitive and the readings could be 
repeated very accurately. 
The cathetometer used in reading the heights of the liquid in the U-tubes was 
of a standard pattern made by the Cambridge Scientific Instrument Company and 
capable of reading to of a millimetre. 
Stream-line Flow. 
The determination of tlie relation of slope to pressure, for vater in stream-line 
motion floydng through tubes of more than capillary size, is rendered somewhat 
difficult because of the smallness of the difference of pressure required to produce the 
flow. The difference may be increased by using a long length of pipe, or by using 
apparatus of extreme accuracy. The disposition of the permanent apparatus in the 
laboratory prevented tlie use of a pipe more than G feet in length between the 
pressure chambers; and at first considerable difficulty was experienced in obtaining 
