88 NOTES ON MAJOR COATES' DISCOVERY. 



but the latter speed would, he thinks, erode the banks of 

 an unpaved channel. The speed of two and a-quarter feet a 

 second with twelve square feet cross section gives a discharge of 

 twenty-seven cubic feet, or 168 gallons a second, but velocity 

 and consequent amount of delivery falls off rapidly as the mean 

 depth diminishes. 



With regard to the water supply itself, at Compton there is a 

 spring of water, excellent in quality. The volume is not now 

 large, but in all probability in Roman times, before the land was 

 cleared, the springs were fuller and the rainfall greater ; but, if this 

 was not the case, a large supply of water could be obtained from 

 the rainfall on the slopes. If the rainfall in those days averaged 

 only thirty-six inches in the course of the year, one square mile 

 would yield nearly forty-two million cubic feet of water, i.e., 260 

 million gallons. If discharged continuously with no waste, this 

 would give a supply of eight gallons a second ; only a fraction of 

 this would be actually delivered, but a much larger proportion of 

 the rainfall would be collected on a slope with watertight strata 

 underlying it than on more porous soil. 



As to the castellum, or rather castella, the reservoirs which 

 directly supplied the town, for Vitruvius mentions three, Major 

 Coates in his paper expressed the opinion that the depression 

 now occupied by the public gardens was the most likely spot for 

 the reservoirs. Those who, like myself, examined the spot care- 

 fully for traces of the Roman fortification of the town before the 

 gardens were laid out, will remember that the tops of two of the 

 valla, somewhat worn down, were clearly apparent in the field 

 which is now the northern end of the gardens, and that these 

 terminated abruptly near the hedge between the two fields into 

 which the site of the gardens was then divided, and that the 

 second field was on a much lower level. South of the latter field 

 Mr. Cunnington, I believe, again found the vallum and fossa, 

 and I am told that the drawing of a section of them is now in 

 the County Museum. 



Any member of the Field Club who may wish to examine the 

 water course where it is to the eye most perfect should examine 



