626 



Obstructions in Draininq- Tiles. 



Cutting. 



LZTt 



=□3 



a few yards above the outfall, and, if it can be conveniently done, 

 at the points of junction of the main drains. These air-traps 



are constructed like or- 

 An-tnip. dinary traps, with two 



or three large tiles placed 

 verticall}^ on a flat stone, 

 or upon a large flat tile, 

 and covered in the same 

 manner ; a little bit of 

 solid work in brick or 

 masonry is placed at the 

 base of these traps. The 

 tiles which discharge into 

 them, in greater or less 

 number, are solidly fixed, 

 and sometimes built in 

 for a short distance to 

 avoid all danger of dis- 

 placement. But contrary 

 to the plan pursued with 

 the ordinary trap, the 

 conducting tile, to which 

 an increased fall is given 

 for a short distance, is 

 placed at a slightly lower 

 Plan. level than the discharging 



tile ; by this device the 

 drain-tiles are cut off from all communication with the outward 

 air, and the desired condition is obtained. 



Ferruginous Obstructions. — The obstructions of this nature are 

 formed by abundant deposits of a muddy or gelatinous consis- 

 tency. Their tint varies from deep red to faded ochreous yellow. 

 When the deposits are formed in calm water you see pellicules 

 of varied hues appear on the surface, which the smallest agita- 

 tion precipitates to the bottom. These deposits rapidly choke 

 tiles of any dimensions, and completely stop the flow of the 

 water. The waters in which these deposits are formed are those 

 especially which spring from lands rich in oxide or in sulphate 

 of iron, from bogs, from peaty grounds, and from lands exposed 

 to infiltrations from woods placed on a higher level. The pro- 

 ducts designated by the names crenic and apocrenic acids play 

 an important part in the production of these deposits. The study 

 of them in a purely chemical point of view merits attention, and 

 I propose to undertake an examination of them as soon as a 

 favourable opportunity permits me to collect a mass of materials 

 sufficient for the inquiry. For the present, in a practical point 

 of view, the following facts will suffice : — The composition of 



