VOL. XXXIV.J PHILOSOPHICAL TRANSACTIONS, 1 3Q 



Now when the water runs from a to b, the first terrel d will catch the air as 

 it runs, so as to let it out at i, if the cock h be open, sometimes without going 

 to G or to c. But if the cock had not been opened, till the water had passed 

 through the part ab of the pipe, the air would lodge in the space cc, and be 

 discharged on the opening of the cock. After the cock has been shut, when 

 no more air comes, and water succeeds, after some time, air will extricate it- 

 self out of the water, and come up to cc ; or if it comes from the parts of the 

 pipe towards b, it will rise contrary to the current of the water, quite up to c, 

 and so go out at the pipe eh, when the cock is opened again. 



As, after the first discharge of the air, it cannot be known when more air is 

 got into the pipe, unless by opening the cock, which would require one man to 

 attend each cock constantly, and occasion a waste of water at every turn of the 

 cock, unless when air happens to be in the pipe ; it was proposed to contrive a 

 valve that should open to let out the air, and shut again when the water came ; 

 and an inverted brass clack or valve shutting upwards, and falling down by its 

 own weight, with cork fixed to the under-side of it to help it to rise when the 

 water came, was mentioned as fit for the purpose by some of the persons pre 

 sent. But we rejected that proposal ; because, when such a valve has been shut 

 some time, if air should extricate itself from the water, it would be dense air, 

 whose force being equal to that of a column of water 30, 6o, 80 or more feet 

 in height, it would keep the valve shut as well as the water did before, though 

 the air at first could not shut the said valve. 



At last, after several thoughts, we contrived a machine which exactly answers 

 the purpose, and is very simple ; therefore it will be of general use. The 

 description of it is as follows : in fig. 6, g is a section of the main or conduct- 

 pipe, with water up to g, and air above it, ab being a horizontal line touching 

 the top of the said pipe : ehi is the leaden pipe described above, and marked 

 with the same letters as in fig. 5, reaching from the pipe in the street to the 

 side of a house, or to the side of one of the posts set up to keep off coaches 

 from the foot-way. The machine is the box k, made of cast-iron, fixed to the 

 leaden pipe at i, with a thin door of plate-iron, moving on hinges, and made 

 to lock at D. This box stands in the street out of the way of passengers, with 

 its bottom fixed to a plank in the pavement, so as not to be damaged by a 

 small shock or any chance blow. 



The several parts of the machine are the following. In fig. T , nn is an iron 

 plate, about an inch thick, with 4 holes at 1,2, 3, 4, of about an inch diameter, 

 quite through the plate, to let through 4 screws, such as a ; oo is a face, or 

 flat ring, raised out of the whole stuff', and prominent about J- of an inch, 



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