THE LAW OF RESISTANCE IN PARALLEL CHANNELS. 
959 
the control cock from the main, one of these being furnished with an indiarubber 
mouthpiece with a screw clip upon it, so that part of the water which passed the 
cock might be allowed to run to waste, the other branch being connected with the 
lower end of a vertical glass tube, about 6 millims. in diameter and 80 inches long, 
having a bulb about 2 inches diameter near its lower extremity, and being closed 
by a similar bulb at its top. 
This arrangement served as a delicate pressure gauge. The water entering at the 
lower end forced the air from the lower bulb into the upper, causing a pressure of 
about 30 inches of mercury. Any further rise increased this pressure by forcing the 
air in the tubes into the upper bulb, and by the weight of water in the tube. During 
an experiment the screw clip was continually adjusted, so as to keep the level of the 
water in the glass tube between the bulbs constant. 
26. The resistance gauges .—Only the last 5 feet of the tube was used for measuring 
the resistance, the first 10 or 11 feet being allowed for the acquirement of a regular 
condition of flow. 
It was a matter of guessing that 10 feet would be sufficient for this, but since, com¬ 
pared with the diameter, this length was double as great for the smaller tube, it was 
expected that any insufficiency would show itself in a greater irregularity of the 
results obtained with the larger tube, and as no such irregularity was noticed it 
appears to have been sufficient. 
At distances of 5 feet near the ends of the pipe, two holes of about 1 milliin. were 
pierced into each of the pipes for the purpose of gauging the pressures at these points 
of the pipes. As owing to the rapid motion of the water in the pipes past these 
holes, any burr or roughness caused in the inside of the pipe in piercing these holes 
would be apt to cause a disturbance in the pressure, it was very important that this 
should be avoided. This at first seemed difficult, as owing to the distance— 5 feet— 
of one of the holes from the end of pipes of such small diameter the removal of a burr, 
which would be certain to ensue on drilling the holes from the outside, was difficult. 
This was overcome by the simple expedient suggested by Mr. Foster of drilling holes 
completely through the pipes and then plugging the side on which the drill entered. 
Trials were made, and it was found that the burr thus caused was very slight. 
Before drilling the holes short tubes had been soldered to the pipes, so that the holes 
communicated with these tubes; these tubes were then connected with the limbs of a 
siphon gauge by indiarubber pipes. 
These gauges were about 30 inches long ; two were used, the one containing mer¬ 
cury, the other bisulphide of carbon. 
These gauges were constructed by bending a piece of glass tube into a U form, so 
that the two limbs were parallel and at about one inch apart. 
Glass tubes are seldom quite uniform in diameter, and there was a difference in the 
size of the limbs of both gauges, the difference being considerable in the case of the 
bisulphide of carbon. 
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