70 THE FOOD OF PLANTS 



Turn the brass stud slowly round, and bring the beam 

 gently on to its supports again. It is now off the knife 

 edges, and can no longer swing to and fro. It should 

 always be in this position when not in use, and when any- 

 thing is going to be placed on or removed from either pan. 



Now open the glass front of the balance, dust the 

 pans with a soft brush, and again turn the brass stud. 

 The beam commences to swing to and fro, and the 

 pointer to move backwards and forwards in front of 

 the little ivory scale. 



If the balance is equally weighted on both sides, the 

 pointer will swing to about equal distances on both sides 

 of the middle line of the scale. This can be accurately 

 tested by noting the divisions on the scale. As the 

 balance is slowly coming to rest, the pointer will swing 

 a little less each time, and therefore in order to tell if 

 the weights on each pan are exactly equal we should 

 have to note down each swing and take the mean. 



For most purposes it is sufficient to note that the 

 pointer is very nearly equal in its swings on both sides 

 of the dividing line. For instance, it may swing 4 

 on one side, 3 on the other side, 2 on the first side, 

 and so on. Evidently the beam is equally balanced 

 but slowly stopping. 



If you wish to increase the swing of the beam gently 

 fan the air with your hand near one of the pans. While 

 noting the swings of the beam, close the glass front, 

 otherwise currents of air entering the balance case will 

 disturb the weighing. 



As a rule, on trying the balance you will find the 

 equilibrium imperfect. You will notice on the end of the 

 beam a little nut running on a screw. By screwing this 

 in or out the balance can be perfectly adjusted. Lower 

 the beam, wind in or out as the case may be the little 

 nut, raise the beam again, note the swings, and repeat 

 until the balance is in equilibrium. You are now ready 

 to weigh something. 



