SCIENCE AND PRACTICE. 351 



the idea of testing the single knots of his cable, destined for 

 submersion between England and Holland, under the pressure 

 of a column of water about equal to that which would press 

 upon it after being paid out. He feared especially to sub- 

 merge a cable without this provisional test, as, in case of 

 the existence of air-bubbles or small fissures in the dielectric, 

 the weak places might otherwise remain undiscovered until 

 the water, under the greater pressure, forcing its way in, 

 would develop them, when too late to remedy the evil. For 

 the accomplishment of his design, he took his core in coils in 

 a vessel to sea off Sunderland, and there, retaining one end 

 of each coil on board, having insulated the other, sunk them 

 to the bottom, and tested their insulation while there by 

 means of a galvanometer and battery. 



This method of testing under high pressure entailed, 

 however, considerable expense and trouble, which, combined 

 with the disinclination of some of the coils to sink, induced 

 Mr. Heid to construct cylinders in which he could place 

 them, and, by means of a system of force-pumps, subject 

 them to a considerable pressure. Another advantage accru- 

 ing from the employment of tanks, instead of sinking the 

 coils in the sea, was the opportunity it gave of subjecting 

 the insulated wire to a vacuum before the application of the 

 pressure. This the inventor believed to be essential, as air- 

 bubbles which, like elastic balls, might not be broken by 

 any pressure, however great, uniformly surrounding them 

 would be burst by the internal pressure when put into a 

 chamber from which the air was exhausted ; and, further, 

 that, should fissures occur in the envelope, the air would be 

 expelled from them, and the water afterwards compelled to 

 enter. 



The first pressure tanks of any importance were those 

 erected on the premises of the Wharf Road Gutta-percha 

 Works, and employed in testing the cores of the Toulon- 

 Algiers and Malta- Alexandrian cables. These cores were 

 subjected to a partial vacuum, and to a pressure of 600 pounds 

 per square inch. 



Subsequently Mr. Reid constructed another tank, put up 



