25 



4. Special Measuring Devices. 



Fig. Sl.^Grant-Michell meter; capacity 100 cubic feet per minute. 



The Grant Michell meter is known as the Australian meter, having 

 originated in that country. It consists of a four bladed fan fastened to the 

 lower end of a vertical spindle which transmits the revolution of the fan to 

 a gear box at the upper end. This mechanism is suspended from a cast iron 

 bracket over a wrought iron orifice plate placed below the canal bed and 

 built or bolted down into the downstream part of a box divided into two 

 parts by a baffle wall open at the bottom and extending above the water 

 surface. The water passes down through the opening formed by the 

 baffle wall and the box, then flows upward through the orifice and imparts 

 a rotary motion to the fan. The gear box forms the recording device, which 

 consists of a series of dials giving a continuous record in acre inches and 

 fractions, or in cubic feet. The fan, spindle and gear box are removable 

 and portable and can be used for several boxes. The discharge depends on 

 the size of the orifice plate and on the difference in elevation in the water 

 surface upstream and downstream. The size of the meters is generally 

 based on a 3 inch loss of head, but may be designed for less. The orifices 

 range from 9 to 40 inches in diameter and are used for discharge of 1 to 20 

 cubic feet per second. The serious objection to this meter is the high cost, 

 ranging from $60 for the smaller one to $250 for the larger one. These 

 prices are the catalogue prices of Geo. Kent, Ltd., 199-204 High Holborn 

 Street, London. 



5. Measurement of Discharge by Obtaining Velocity of Flow and Cross 

 Section of Flume or Canal. 



This method is best adapted to the measurement of large volumes of 

 water when the installation of a weir is not feasible because of lack of 

 grade or because of the large volume of water. The method depends on the 

 principle that the discharge in cubic feet per second is equal to the area of 

 the cross section of the stream in square feet multiplied by the velocity in 

 feet per second. The following principles will give a better understanding 

 of this method of measurement. 



1st. The velocity in a canal varies with the form of the channel, the 

 smoothness of the sides and bottom and the grade. 



