ON KEEPING MEDUSAE ALIVE IN AN AQUAIIIUM. 



177 



downward plunge the plate made in the early experiments it was called 

 a " plunger," and this name I have retained for the want of a simpler 

 one. (Fig. 1.) 



The plunger consists of a flat glass plate with a small hole in the centre; 

 through the hole passes a glass rod which is suspended to one end 

 of a long wooden rod, the " beam." The glass rod has a knob at the 

 bottom upon which the plate rests, and it is slightly bent so as to give 

 a slope to the plate. The sloping of the plate prevents the medusae 



Fig. 1. Bell-jak with Glass Plunger. 



being caught between the plate and the surface-film when the plunger 

 moves up. 



The beam rests near its centre on a pivot, like the beam of a balance, 

 and at one end is suspended the plunger, and at the other end a small 

 bucket made of tin, fitted with a large siphon. A rubber-tube conveys 

 a constant ilow of water (from the fresh-water supply) into the bucket 

 which, when full, quickly empties itself through the siphon. When 

 the bucket has been emptied by the siphon, the plunger is at the 

 bottom of the bell-jar, as the plunger end of the beam is heavier than 

 the bucket end. The plunger need only be a little heavier than the 

 bucket; and the weight can be easily regulated by means of a bottle 

 containing shot, attached to the beam. 



