66 



MR. H. J. MARTEN ON SOME 



weighed 16 lb. 12 oz., thus giving the weight of a cubic foot of 

 saturated stone, as lying in the quarrj^ and in the apron of the weir, 

 as 134 lb. ; and the weight of each stone when placed in the weir 

 was estimated on this basis. The stones taken out of the weir were 

 also weighed in a saturated state. 



The pebbles were probably rolled by the current in flood-times up 

 the somewhat steep up-stream face of the weir, and deposited, as 

 previously described, in the hollows and joints of the stones forming 

 the flat apron or down-stream side of the weir (see figs. 7 & 8). 



Figs. 7 & 8. — Plan and Section of the Holt Weir on the Severn. 



« ! I I I Wllllltllim 



Scale Jor Weir 



When the weir was first constructed, the current on the up-stream 

 face was so strong that boulders of considerable size were driven 

 over the crest. 



The average quantity of water passing over each square foot of 

 the surface of the stones composing the apron of the weir is esti- 

 mated at about 2000 gallons a minute ; each gallon of water has an 

 average scrubbing-velocity of from 12 to 15 feet per second, and this, 

 acting on the small pebbles, will give some idea of the forces at 

 work for the 43 years during which the stones were in the weir 

 apron. 



There are not many instances in which the specific facts relating 

 to the action of water and pebbles upon a certain class of stone can 

 be so accurately ascertained as in the present case ; and the writer 

 has therefore ventured to submit a record of them, in the hope that 

 they may be of use to those who may have occasion to investigate 

 the periods of time likely to be occupied in changes resulting from 

 the abrading action of water and pebbles upon the rocky beds of 

 streams and rivers and over waterfalls into ravines. 



