52 EXPERIMENTS WITH PLANTS 



adjusted by means of the nut at the bottom of the 

 eye-bolt). Arrange the weights as desired, and pom- 

 water into the pan in which the cups stand. Observe 

 from time to time; if the lever rises, move the weights 

 away from the cups until it sinks to the level it occu- 

 pied at the beginning. If, on the contrary, it falls, 

 watch it, for it will probably sink for a time and then 

 begin to rise, in which case add more weight. If 

 after forty-eight hours no additional rise is apparent, 

 we may consider the experiment at an end and may 

 then proceed to calculate the pressure exerted. This 

 we must do for each weight separately. Thus, for the 

 first weight divide the distance a c by a b and multiply 

 by the weight; for the second, divide ad by a 6 and 

 multiply by the weight. Add these products together, 

 and then add the pressure exerted by the bar itself, 

 which may be calculated as follows: 



Pressure = ('^ f) ^ (weight of bar) 

 (a b) X 2. 



Thus, if the length of the bar {a f) is thirty- six 

 inches, the distance a i two inches and the weight of 

 the bar eight pounds, the equation is 



Pressure = „ ' „ = 72 pounds. 

 2x2 



The results obtained by the above methods are only 

 rough approximations. It should be remembered that 

 while, theoretically, one layer of seeds can exert as 



