278 



NA TURE 



\_yan. 20, 1 88 7 



cylinder. By means of a pin and a pair of sliding blocks 

 running in circular grooves cut on the inner surfiice of, 

 and concentric with, the fixed cylinder, these shutters are 

 drawn out and in from the revolving cylinder so as always 

 to keep in contact with the fixed one. During one revolu- 

 tion these shutters sweep through the meter a volume of 

 air about '17 cubic feet. 



This rotation is reduced three times by worm-gearing 

 in being transmitted to the counter-box, so that a single 

 dial with two concentric circular scales, which are read 

 by two fingers like the hour- and minute-hands of acommon 

 clock, is sufficient to register up to a million cubic 

 feet. Fig. i shows this meter. It is driven by a small 

 difference of air-pressure betiteen the inlet and outlet. 



The Company intend to charge at the rate of ^d. per 

 1000 cubic feet at standard pressure of 45 pounds per 

 square inch. If the air were used in an engine without 

 expansion, without clearance space, and without back 

 pressure above the atmosphere, this would correspond to 

 a cost per hour per indicated horse-posver of 



6oX33°°°X5_^i.„pence. 



144 X 45 X 1000 

 Under the conditions of actual practice the writer calcu- 

 lates that at the above rate of 5^/. per 1000 cubic feet, 

 assuming intelligent and economical management, each 



Fig. 2. 



indicated horse-power will cost per hour from id. down to 

 as low as \\d., excluding cost of engine attendance and 

 depreciation, and interest on first cost of engine. 



The standard pressure at which the air is sold at the 

 above price being 45 pounds per square inch, a reduction 

 of price per cubic foot has to be made if the pressure of 

 the supply be less than this pressure. This is effected by 

 introducing a variable velocity-gear between the volume- 

 meter and the dial-counter. This arrangement is shown 

 in Fig. 2. 



The rotation is transmitted to a small roller on a spindle 

 capable of sliding in its bearings parallelly to its own 

 axis. It drives a disk on the counter-arbour by rolling 

 contact. The end of the roller-spindle is linked to the 

 end of the tube of a Bourdon pressure-gauge. As the 

 pressure rises, the roller is thus pushed nearer the centre 

 of the disk, and gives this disk, therefore, an increasing 

 fraction of a revolution per revolution of the roller. The 

 roller really lies between two disks, but the one is " idle " 

 and serves simply to support the roller in pressing against 

 the driven disk. 



This integrator is wholly wrong in principle, and it is 

 badly designed in detail. The roller has a rubber tyre 

 round it, and therefore touches the disk at different radii, 

 and thus must rapidly wear away, owing to the want of 



pure rolling action at one or other side of its tread. The 

 wearing might not be of much consequence in itself, ex- 

 cept that it gradually vitiates the accuracy of the indica- 

 tion ; and besides, the velocity ratio is uncertain because of 

 the contact taking place over a perceptible range of radius. 

 There ought 10 be an idle roller between the disks opposite 

 the driving roller, and both disks ought to be pressed in- 

 wards by springs, instead of one onl)-. But the cliief 

 defect is in the principle of the construction, which does 

 not make the dial-indication proportional to /'Kas it 

 ought to do. If R^ be the disk radius at which the roller 

 would stand when zero pressure existed in the Bourdon 

 tube, and if C be the inward movement per pound per 

 square inch rise of pressure, and if r be the radius of the 

 roller, then at pressure /■" the contact radius on the disk 

 will be R^ - CP, and the fractional revolution of the disk 



per revolution of the roller is 



This is notpro- 



portional to P as it ought to be. Its differential co- 

 efficient with respect to P should be constant, whereas it 



is really — — ^-— ,. The converse gearing ought to 



be substituted ; that is, the voUime-meter should be 

 geared positively with the disk, and the disk should drive 



the roller, the point of contact for zero pressure coinciding 

 with the centre of the disk. It also seems a pity, when 

 a Bourdon tube that measures the pressure exists in any 

 case in the meter, that its measurement of the pressure 

 should not be made visible by the simple addition of a 

 pointer and graduated dial. 



The registrations of all the meters in the whole district 

 are telegraphed to the central station and added up on 

 one large central counter, so that the engineers in charge 

 may have means of continually comparing the actual 

 consumption with the duty of the engines, know-n from 

 ordinary engine continuous counters, and of detecting any 

 serious leakage that might occur in consequence of break- 

 age of a main or branch pipe. The telegraphing apparatus 

 is shown in Fig. 3. The counting disk is divided into ten 

 equal divisions, each representing 1000 cubic feet, by 

 small met'il projections. As these come successively 

 underneath a contact-maker, they allow the passage of a 

 current, which moves the finger of the central counter 

 through a corresponding division. One main wire, with 

 branches to the separate meters, is sufficient for the whole 

 district, the earth return being used. As the counter-disk 

 moves slowly, special means must be taken to break the 



