475 



merits and class-room, along with the assistance of one of his 

 establishment, I am enabled to give the few following results of ex- 

 periments tried on brick-clay from Portobello, sea and fresh- water 

 sand, rounded pebbles about the size of peas, and common vegetable 

 soil. 



1. The brick clay, in its natural moist state, had a specific gravity 

 of 2*05 ; and water passing over it for half an hour at a rate of 

 128 feet in the minute, which was the greatest velocity I could con- 

 veniently obtain, made no visible impression on the clay. When 

 this clay was mixed with water, and allowed to settle for half an 

 hour, it required a velocity of fifteen feet in the minute to disturb it. 

 This mud sank in water at a rate of 0*566 feet in one minute, but 

 the very fine particles were very much longer in subsiding. 



2. The fresh-water sand, which sank in water at an average rate 

 of 10 feet in the minute, required a velocity of 40 feet in the 

 minute over the bed to disturb it. 



3. The sea-sand sank 11 '707 feet in one minute, and was moved 

 over the bed by a velocity of 66*22 feet per minute. 



4. The rounded pebbles, about the size of peas, which sank at the 

 rate of 60 feet in one minute, were rolled over each other by a ve- 

 locity of 120 feet in one minute. 



5. The vegetable soil, being a mixture of different kinds of par- 

 ticles, it was difficult to determine the rate of sinking. The very 

 fine particles were swept away by a velocity of 2*45 feet in the 

 minute, and those that were disturbed by a velocity of 33^ feet in 

 the minute, sank at a rate of 0*98 feet per minute only. All the 

 vegetable soil was swept away when the velocity of the water over 

 the bed was increased to 50 feet in the minute, and the particles of 

 sand left, of which nearly one-half of this soil consisted, were set in 

 motion by this velocity. This sand, however, only sank at a rate of 

 5*62 feet in one minute. 



From the above it is evident that the velocity of water over the 

 beds of canals, and water-courses for irrigation, unless protected by 

 a pebbly bed, should never exceed half-a-mile an hour, otherwise 

 the fields irrigated will be covered with a stratum of sand. 



These experiments were made with water seldom exceeding half 

 an inch in depth, and the float was within J or J of an inch of the 

 bottom. The time was measured by an instrument which indicated 

 tenths of seconds. 



