MATERIAL AND METHODS. 9 



parts are counterbalanced by the weights (W W) , which may be so 

 adjusted that the floors move up and down at a very light touch. The 

 extent and place of the excursion of the levers are controlled by check 

 blocks, shown in plate A, figs. B and C, attached to the outside of the 

 chronograph box. The feed dish (FD) is on a little shelf attached to the 

 inside of the box, and thus independent of the movable parts, as is also 

 the water bottle. 



Fig. D. 



In use, the weights are so adjusted that the empty nest (N) with its 

 floor (NF) is raised to its upper limit, but may be depressed by a weight 

 of only 2 to 3 grams; the main floor (MF), on the other hand, requires 

 about 10 grams to depress it. 



When, therefore, there is no mouse present, both parts of the floor 

 (NF and MF) are up, and the pointers (NP FP, fig. E) are down. If, 

 under these conditions, the chronograph drum is set in motion, the two 



