21 



The disadvantages are: 



1st. The device is not adaptable to large volumes of wtaer because the 

 required length of the orifice may be too great. 



2nd. Unless the conditions necessary for accurate measurements are 

 carried out, the results obtained may' be very inaccurate. 



The device consists of a board or thin wall in which is made an orifice, 

 which conforms with the conditions necessary to give the volume as defined 

 by law. The board may be set directly across the ditch or it may be placed 

 in a short section of flume or in a box. 



The controlling conditions to give measurements in British Columbia 

 miners' inches are given in the Water Clauses Consolidation Act of 1897. 

 Briefly stated the miners' inch orifice must be 2 inches high and the head 

 on the upper side of the orifice must be 7 inches; this gives 8 inches on the 

 center of the orifice. The Act states that the orifices must be made in a 2 

 inch board. This thickness is liable to give inaccurate results, unless the 

 four edges of the orifice are bevelled outwards so as to give sharp corners. 

 To obtain accurate results the jet coming through the orifice must touch 

 only the upstream edges and clear the downstream edges so as to discharge 

 freely into the air. With a board 1 inch thick, if the corners are sharp, it 

 is not necessary to bevel the edges for the water jet will clear the outer 

 edges. It is, however, preferable to either bevel the edges or use thin 

 metal plates. The orifice must be made of such length that it will measure 

 the maximum amount of water desired. For instance if this volume of 

 water is 100 miners' inches, the orifice 2 inches high must be 50 inches 

 long. To obtain a smaller volume the orifice must be adjustable by means 

 of a sliding gate. 



To measure a large volume of water the length of the orifice will be 

 excessive. To avoid this it may be desirable to use an orifice 4 inches high 

 instead of 2 inches. This will make the necessary length only one-half the 

 length required for the 2 inch height and the accuracy of the device will 

 not be affected to any great extent provided the orifice is made in a thin 

 plate. In either case the head on the center of the orifice must be 8 inches. 



To obtain accurate results at least 6 inches must be allowed on the up- 

 stream side of the board from the lower edge of the orifice to the bottom of 

 the ditch or floor of the box into which the board is placed, and at least 2 

 to 4 inches from the ends of the orifice to the sides of the ditch or box. On 

 the downstream side the jet should discharge freely into the air. 



The form of construction of the miners' inch measuring device will de- 

 pend on the purpose for which it is used. It may be used to measure the 

 flow in a ditch or flume in which case it may be given the form shown in 

 Fig. 15 or it may be used to measure the water carried by a pipe under no 

 pressure and is then built as shown in Fig. 16. Where the pipe is under 

 pressure the miners' inch box can be built around the valve in the same 

 manner as the weir box shown in Fig. 11, using a miners' inch orifice plate 

 in the place of the weir plate. 



Miners' inch box placed in canal or ditch. 



The form of miners' inch box to use for measurement of water carried 

 by a flume or ditch is shown in Fig. 15. This box consists of a short sec- 

 tion of flume with the miners' inch board placed at the downstream end. 

 The box may be made as the weir box shown in Fig. 7 and connected with 

 the earth ditch by similar wings and cut off apron. The miners' inch ori- 



