HYDROSTATICS. 137 



_ - the air-bubble, a. In whaterer position the tube 



mav be placed, the bubble of air will rest at the high- 

 est point If the two ends of the tube are level, or 



/' V ^ -V cot UViUU -tA LUC L»V CLU^O \JX lliC lUl-'t' di V A^ * ^-J, yJL 



\2 t^ perfectly horizontal, the air-bubble will remain in 



the center of the tube ; but if the tube inclines ever 

 80 little, the bubble rises to the higher end. For practical use the glass-tube 

 is inclosed in a wood, or brass case, or box. 



309. The method of conducting' a canal through a countrv, 

 L pon what pnn- , , - , . , . . , , . , , , ,' 



ci.jle are canals the surface of which is not pertectlv horizontal, or level, de- 

 et'.istructed and ^Qjxda upon this same property of hquids. In order that boats 

 may saU with ease in both directions of the canal, it is neces- 

 sary that the surface of the water should be level. If one end of a canal 

 were higher than the other, the water would run toward the lower extremity, 

 overflow the banks, and leave the other end dry. But a canal rarely, 

 if ever, passes through a section of country of any great extent, which is 

 not incUned, or irregular in its surface. By means, however, of expedients 

 called Locks, a canal can be conducted along any declivity. In the forma- 

 tion of a canal, its course is divided into a series of levels corresponding 

 with the inequaUties of the surface of the country through which it passes. 

 These levels communicate with each other by locks, by means of which 

 boats passing in any direction can be elevated, or lowered with ease, rapidity, 

 and safety. 



Fig. 124. Fig. 124 represents a section of 



a lock, and Fig. 125 the constnic- 

 tion of the Lock Gates, The sec- 

 tion of Fig. 125 represents a place 

 •where there is a sudden fall of the 

 ground, along which the canal has 

 to pass. A B and C D are two 

 gates which completely intercept 

 the course of the water, but at the same time admit of being opened and 

 closed. A H is the level of the water in that part of the canal lying 

 above the gate A B, and E F and F G the levels below the gate A B. The 

 part of the canal included between two gates, as E F, is called a lock, because 

 when a vessel is let into it. it can be shut by closing both pair of gates. If 

 now it is required to let a boat down from the higher level, A H, to the lower 

 level, E G. the gates C D are closed tightly, and an opening made in the 

 gates A B (shown in Fig. 125), which allows the water to flow gradually from 

 A H into the lock A E F C, until it attains a common level, H A C. The 

 gate A B is then opened, and the boat floats into the lock A B C D. The 

 gates A B are then closed, and an opening made in gates C D, which allows 

 the water to flow from the space A E F C, until it comes to the common 

 level, E F G. The gate C D is then opened, and the boat floats out of 

 the locks into the continuation of the canal. To enable a boat to pass from 

 the lower level, E F G, to the superior level, A H, the process here described 

 is reversei 



•^ H | A C^ 



