482 U. S. BUREAU OP FISHERIES 



SELF-CENTERING TUBE 



In place of the glass tube held rigidly in position by the fittings in 

 the closed top of the McDonald jar, through which water is supplied, 

 a metal tube (fig. 8) is substituted in the open-top jars. This tube, 

 which is usually of tin, is 18 inches long and % inch in diameter. 

 Fastened to one end are four triangular pieces of tin, on which the 

 tube rests in an upright position. The shape of these "feet" is such 

 as to raise the end of the tube about 34 inch from the jar bottom, 

 permitting an even escape of water and causing it to rest automat- 

 ically at the lowest point in the center of the jar. The rubber tube 

 from the water supply is inserted in the upper end of the metal tube, 

 and the flow of water is controlled by the petcock. In phice of this 

 metal tube a glass tube, connected with the water-supply petcock by 

 a rubber tube, may be used, in which case the desired level of the 

 tube in the jar is regulated by the length of the rubber-tube 

 connection. 



Another type of tube has been pronounced as more efficient. It 

 is approximately 18 inches long and % inch in diameter; at the 

 bottom is a cone 3}/2 inches long and 23^ inches in diameter at the 

 base. Three legs are soldered to the cone to keep it about J/4 inch 

 from the sides of the glass jar and to permit the water to escape. 

 The theory is advanced that this cone reduces the velocity of the 

 water at its outlet and does not cause the eggs at this point to 

 be set in rapid motion, as is done when the straight tube is used. 

 The velocity of the water apparently does not affect the eggs while 

 they are in the tender stage, and for this reason the cone tubes have 

 not come into general use. 



PREPARING JARS FOR OPERATION 



In preparing the McDonald jar for work the side tube is fitted first. 

 The glass tube should be wet, the gum washer slipped on the tube 

 about an inch from the end and introduced into the opening. Hold- 

 ing the tube perpendicularly to the face of the plate, press fairly on 

 the tube, ana the washer, rolling on itseK, will fall into the seat pro- 

 vided for it. Screw on the binding ring, and test by seeing that the 

 tube slides freely back and forth in the stuffing box; if not, it should 

 be refitted with a heavier or lighter washer, as may be required. 

 Glass tubes of absolute uniformity in size can not be procured. 

 Water is the only lubricant that should be used about the jar fittings. 



The jar after being washed clean is filled with fresh water. A 

 shallow tin funnel with a perforated rim is inserted, so that the water 

 will stand as high in tlie funnel throat as possible. 



The requisite number of eggs being in the jar, it is put in position 

 and closed, care being taken that both tlie inlet and outlet tubes slide 

 freely in their stuffing boxes. If the tubes become gummed, let water 

 trickle down around the binding screws. To close the jar, turn on 

 the water, place the feed tul)e in the jar, and turn off the water 

 immediately after the feed tube has passed beneath the surface of the 

 water in the jar, thus expelling all the air from the feed tube; other- 

 wise it would rise in bubbles, throwing a portion of the eggs out 

 through the outlet tube. 



