85 SUMMARY OF HYDRAULICS. 



pipe may form either a cylindrical, a conical, or a plane fpiral* 

 and it appears to be uncertain which is the molt advantageous: 

 the vertical pipe fhould be nearly of the fame dimenfions 

 as the fpiral pipe, which may without difficulty be made of 

 wood. 



Screw of Archi- Chapter 21. Of the fcrew of Archimedes, or the water- 

 fnail, and of the waterfcrew. 



The fcrew of Archimedes confifts either of a pipe wound 

 fpirally round a cylinder, or of one or more fpiral excavations, 

 formed by means of fpiral projections from an internal cylinder, 

 covered by an external coating fo as to be water-tight. But if 



The water- the coating is detached, fo as to remain at reft while the fpirals 



fcrew. revolve, the machine is called a waterfcrew. Mr. Eytelwein 



enters into an accurate determination of the effects of thefe ma- 

 chines in given circumftances, and the refults of the theory 



Caution for agree remarkably well with experiment. He obferves, that 



©/Archimedes.* tne ^ crew °f Archimedes fhould always be fo placed, as to fill 

 exactly one half of a convolution in each turn ; and that very 

 unfavourable reports have fometimes been made of the machine 

 from want of attention to this circumftance ;. for when the 

 orifice remains conftantly immerfed, the effect is very much 

 diminifhed : this appears alfo to have happened in fome late 

 experiments in London. Where the height of the water is fo 

 variable as to render this precaution impoffible, Mr. Eytelwein 

 prefers the waterfcrew ; although, in this inftrument, one third 

 of the water generally runs back, and it is eafily clogged by 

 accidental impurities in the water. The fcrew of Archimedes 

 is generally placed fo as to form an angle of between 45° and 

 60° with the horizon ; but the open waterfcrew at an angle of 

 30° only : for great heights, the fpiral pump is preferable to 

 either. 



Bucket wheels. Chapter 22. Of bucket wheels and throwing wheels. 



In the conftruction of wheels for raifing water in buckets, 

 there is little room for refined theory ; whether the buckets be 



Throwing fixed or fufpended on an axis. It is fometimes convenient to 



raife water to the height of 3 or 4 feet by the revolution of a 

 wheel with fimple floatboards ; and fuch a wheel may be either 

 in a vertical or an inclined pofition ; it mult of courfe be in* 

 clofed in a fweep. 



Chapter 23. Of cellular pump? and paternoiter works. 



3 Water 



wheel. 



