88 MADRAS FISHERIES BULLETIN VoL. XIII, 



as not to be ordinarily detected or to get into operation very slowly; 

 it is possible that many of the mysterious swells which occur after 

 months of storage may be due to this slowly operating cause. 



107. Exhausting may be variously effected, e.g., by placing the 

 cans, properly arranged in wire baskets, in boiling water; the cans 

 are provided with a pin hole vent in the cap, and after a few 

 minutes sharp boiling, the baskets are removed and the vents 

 sealed with a drop of solder ; this is known as ' tipping ' the cans. 

 This method is generally used for tall cans, so that the vents 

 may be above the water. But cans of all sizes and shapes with 

 open vents may be exhausted in a steam retort; the trollies of cans 

 are run into the retort, left there for a few minutes, again run out and 

 at once sealed while hot. In both cases the cans and their contents 

 are very hot and the air is highly expanded, so that if tipping is 

 smartly effected the external air will be quite sufficiently excluded 

 from entrance. In the Beypore cannery and probably in most fish 

 canneries where mechanical exhausting is not practised, the cans 

 of every kind are sealed without a vent, and then submerged 

 entirely in boiling water or placed in the steam retort (at low 

 pressure); when removed (usually slightly convex in shape), the^ 

 are at once pricked with a fine punch so as to form a pin hole vent 

 through which the air and steam escape with a sharp hiss ; they 

 are then at once tipped by the solderer who immediately follows 

 the venter. In large canneries a conveyer takes the vented cans 

 slowly and continuously through a steam box on emerging from 

 which the vents are sealed. 



In the Beypore cannery exhausting is practically continuous 

 with testing by simply bringing to the boil the water of the testing 

 vat as soon as the cans under test have satisfactorily passed. 



108. The most modern method is exhaustion by mechanical 

 means : an exhausting vessel usually of corridor shape, is so 

 arranged that the vented cans pass through an air-lock into the 

 exhauster which is kept at a considerable vacuum by connexion 

 with an air pump and consequently sucks out the air from the cans 

 through the vent ; at a certain point the cans pass slowly under an 

 electrically worked soldering iron within the exhauster and the 

 tipper is thus enabled to seal the cans while still in the vacuum. 



109. Whatever the mode adopted the exhausting should be such 

 that so much of the included air is expelled or sucked out that the 

 tins when processed shall be subjected only to moderate internal 



