MANUAL OF FISH-CULTURE. 153 



In reconnecting, tlie air may be expelled with the metal top screwed 

 down in position. To effect this, draw both glass tubes up to the top of 

 the jar and turn on a full head of water, when the air will be forced out 

 in bubbles above the eggs, the bubbles escaping through the outlet 

 tube. The central tube is now restored to its former position. The 

 automatic action permits entire separation of bad from good eggs, 

 though some days may be required to accomplish the full result. The 

 dead become lighter from gases arising from decomposition. A net, 

 small enough to easily enter the mouth of the jar and fixed to a handle 

 several inches longer than the jar, is convenient for removing particles 

 of foreign matter. 



Shad eggs are semi-buoyant, and those which Avill not rise commence 

 himi^ing on the third or fourth day. The usual period of hatching is 

 from 6 to 10 days, sometimes longer, according to temperature of water, 

 but with high temperature they will hatch in 3 days. Fry hatched in 

 less than 5 days are usually, though not always, weak. In general, the 

 l)eriod of incubation varies inversely with the prevailing temperature, 

 but continuous dark and cloudy days will retard and strong light will 

 accelerate development under precisely the same conditions of water 

 temperature, and other circumstances not well understood may also 

 have their influence. 



Fry when hatched are about 0.37 inch long. They have been meas- 

 ured at intervals of from 5 to 15 days, from late in May to the middle 

 of October. Toward the middle of August the rate in growth dimin- 

 ishes. When 9 days old they are about 0.G2 inch long. . Fry 0.5 inch 

 long July 20th were 0.75 inch long 8 days later; on August 14th, 2 to 

 2.25 inches; September 20th, 3 to 4 inches; October 1st, 4 to 4J inches; 

 November 4th, 5 to 7 inches. Some years they grow faster than others, 

 and in some streams more rapidly than in others. From the State fish- 

 ponds at Raleigh, North Carolina, 33 were removed in November, 1884, 

 which measured S to 9 inches. Their usual size in the Potomac in the 

 fall is 3 to 4 inches. 



MEASURING THE EGGS AND FRY. 



To estimate the number of eggs and of the young fry was for years 

 rather a difficult matter to accomplish satisfactorily. The standard 

 made use of at the outset was undoubtedly much too high. The scale 

 most used at present is a light square, made of wood, the longer leg 

 being 15 inches and the shorter 7^ inches long. The material is ^ inch 

 wide and ^ inch thick. The graduations are on the longer leg, and 

 read from the lower end upward. The first line is at a height corre- 

 sponding to the level attained in the jar by a measured half-pint of 

 water, and the succeeding lines are determined by the introduction of 

 additional half-pints of water. When the scale is being constructed, 

 the central glass tube is stopped at the lower end that it may displace 

 an amoun>t of water equal to the amount of eggs it will displace in 



