196 HYDRODYNAMICS. 



where they intersect 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 correct ; but if they do not, then one 

 of the sights must be raised or lowered until it be- 

 comes so. 



In laying out canals and rail-roads, where extreme 

 accuracy is needed, the spirit-level attached to a tele- 

 scope is used. So great is the perfection of this instru- 

 ment, that separate lines of levels have been run with 

 it for sixty miles without varying two thirds of an inch 

 for the whole distance. 



The use of a cheap and simple instrument to determ- 

 ine the position and descent of ditches with ease and 

 precision, before commencing with the spade, will save 

 a vast amount of the trouble and expense which those 

 often meet with whose only method is to " cut and tryP 



SECTION III. 



HYDRAULIC MACHINES. 

 ARCHIMEDEAN SCREW. 



Machines for raising water are of frequent use on 

 every farm. One of the simplest contrivances for this 

 purpose is the Screw of Archimedes. It may be easily 

 made by winding a lead tube around a wooden cylin- 

 der or rod {Fig. 165), in the form of a screw. When 

 placed in an inclined position, with one end in water, 

 and made to revolve, the water resting at the lower side 

 of each turn of the screw is gradually carried from one 

 end to the other, and discharged at the upper extremi- 



