56 VETERINARY HYGIENE 



during heavy rain storms. It is very seldom that drain pipes run 

 full bore probably only in some street drains, and then only in 

 exceptional circumstances. The more nearly full drain pipes are 

 kept running with water or sewage the cleaner they will be, and the 

 less chance there will be of silting and its resultant evil consequences. 

 For the average stable or cow byre it will be found that a 4 or 5- 

 inch pipe will be big enough. 



The Gradient of Drain Pipes. The condition of the interior 

 of drain pipes depends very largely upon the rate at which the 

 sewage passes through them. The velocity of the flow depends 

 primarily upon the gradient at which the pipes are laid. Other 

 influencing factors are : the size of the pipes as has been 

 mentioned; the course taken by them, that is, if it is a straight 

 or a curved one; the number and character of bends and traps; 

 the character of the internal facing of the pipes, as a rough surface 

 naturally causes more friction than a smooth one ; the workmanship 

 displayed in laying the pipes and fittings and the character of the 

 sewage. 



A satisfactory gradient is one which will ensure a steady and 

 even passage of both fluid and solid portions of the sewage, or in 

 other words a self-cleansing velocity. The liquids should not run 

 so slow that the solids have an opportunity to settle out, neither 

 should the flow be too rapid, as in this event the water entering 

 a trap at too great a speed is inclined to empty it by syphonic action 

 or at any rate to leave too small a volume behind to form an 

 effective seal, the nature of which is described below. 



For ordinary house or stable drains a speed of three and a half 

 to four feet per second is satisfactory and this is attained in a 4- 

 inch pipe if there is a gradient of one in forty, and in a 6-inch pipe 

 with a gradient of one in sixty if the pipes are running at about 

 one quarter full. 



TRAPS. A trap is a contrivance for preventing sewer gas escap- 

 ing into the house drainage system, or for preventing any gases 

 generated in the portions of the house drainage system which convey 

 soil water passing into those portions which convey cleaner water, 

 or into the building itself. Every form of trap is a nuisance, but 

 a nuisance which is very necessary. A trap may be so badly con- 

 structed as to be a greater danger in itself than if none were present. 

 On the other hand, a well-designed trap properly laid serves its 

 intended purpose with the minimum of objectionable features. 



Since the earliest days of traps the same fundamental principle 

 for holding back gases has been in use, that of the interpolation 

 of a body of water in the drain between the sewer and the house 



