ARCHIMEDEAN SCREW. , 217 



tersect some distant object ; then reverse the instrument, 

 or turn it end for end., and observe whether the threads 

 cross the same mark. If they do, the instrument is cor- 

 rect ; but if they do not, then one of the sights must be 

 raised or lowered imtil it becomes so. 



In laying out canals and rail-roads, where extreme ac- 

 curacy is needed, the spirit-level^ attached to a telescope, 

 is used. So great is the perfection of this instrument, 

 that separate lines of levels have been run with it for six- 

 ty miles, without varying two-thirds of an inch for the 

 whole distance. 



The use of a cheap and simple instrument to determine 

 the position an-d descent of ditches with ease and preci- 

 sion, before commencing with the spade, will save a vast 

 amount of the trouble and expense which those often 

 meet with vrhose only method is to " cut and try.'''' 



HYDRAULIC MACHINES. 



ARCHIMEDEAN SCREW. 



Machines for raising water are of frequent use on every 

 farm. One of the simplest contrivances, in principle, for 

 this purpose, is the Screw of Archimedes. It may be 



Fi.GT. Ul. 



The Screw of Archimedes 



easily made by winding a lead tube around a wooden 

 cylinder or rod (fig. 241), in the form of a screw. Wl^en 

 10 



