236 SECTION MECHANICS. 



sequently the gas companys go in for wet meters, and will not let you 

 have dry meters if they can help it ; but if you take my advice, go in 

 for dry meters whenever it is possible. 



Now, with respect to water meters. There are various kinds of 

 water meters. First, there is an absolute water meter, in which the 

 water is received into a receptacle, much in the same way as in the 

 dry gas meter, and delivered in the same way, a reciprocating motion, 

 and an arrangement giving the means of measuring it ; just as if you 

 passed water instead of steam through a steam-engine, the number of 

 strokes of the steam-engine would give the quantity of steam passed 

 into the engine, and out again. If you want absolute measurement, 

 that is doubtless the best you could arrange for, but it gives you an 

 intermittent current, and destroys the head of water ; consequently, 

 in all modern water meters the arrangement depends on either the 

 turbine or the screw. To put it in the simplest form, if you had a 

 screw, like a screw propeller, fixed in the middle of a pipe, and re- 

 volving, the number of revolutions of the screw would depend on the 

 velocity of the water going past it, and that would give you a rough 

 measure of the amount, but only a very rough measure, for there is a 

 certain amount of friction caused by turning the screw, and besides that 

 a part of the water escapes through the interstices of the screw without 

 acting upon it, and in that way the screw does not record the quantity 

 of water with any great degree of accuracy. Not only that, but it does 

 not record it with the same degree of accuracy at different speeds, and 

 therefore you are not able to apply an easy or certain correction for it. 

 An improvement of that has been introduced by a more complicated 

 form of screw called the turbine or Barker's Mill, where the water is 

 got under more complete control by directing it against vanes properly 

 arranged and proportioned, and in that way a very accurate water meter 

 can be obtained for all ordinary speeds. There is no doubt that if you 

 were to pass the same stream of water through at the rate of half-a- 

 mile an hour, and then, at some other time, at the rate at which water 

 is driven through the hose of a fire-engine, there would be considerable 

 errors, but with Dr. Siemens' turbine arrangement, under ordinary 

 circumstances, with considerable variation of velocity, there arc 

 practically very small limits of error. It really is a turbine or Barker's 

 Mill arrangement, with proper directive vanes to give compensation 



