Because of the effect of induction the current is much delayed in traveling through a long submarine cable and arrives in waves. Professor Thomson devised his mirror galvanometer to meet this difficulty. This device consists of a large coil of very fine wire, in the center of which, in a small air-chamber, is a tiny mirror. Mounted on the back of the mirror are very small magnets. The mirror is suspended by a fiber of the finest silk. Thus the weakest of currents coming in over the wire serve to deflect the mirror, and a beam of light being directed upon the mirror and reflected by it upon a screen, the slightest movement of the mirror is made visible. If the mirror swings too far its action is deadened by compressing the air in the chamber. The instrument is one of the greatest delicacy. Such was the greatest contribution of Professor Thomson to submarine telegraphy. Without it the cable could not have been operated even for a short period. Had it been used from the first the line would not have been ruined and might have been used for a considerable period. Professor Thomson together with Engineer Bright made a careful investigation of the causes of failure. The professor pointed out that had the mirror galvanometer been used with a moderate current the cable could have been continued in successful operation. Ha continued to improve this apparatus and at the same time busied himself with a recording instrument to be used for cable work. Both Thomson and Bright had recommended a larger and stronger cable, and other failures in cable-laying in the Red Sea and elsewhere in the next few years bore out their contentions. But with each failure new experience was gained and methods were perfected. Professor Thomson continued his work with the utmost diligence and continued to add to the fund of scientific knowledge on the subject. So it was that he was prepared to take his place as scientific leader of the next great effort. X A SUCCESSFUL CABLE ATTAINED Field Raises New Capital--The _Great Eastern_ Secured and Equipped--Staff Organized with Professor Thomson as Scientific Director--Cable Parts and is Lost--Field Perseveres--The Cable Recovered--The Continents Linked at Last--A Commercial Success--Public Jubilation--Modern Cables. The early 'sixties were trying years for the cable pioneers. It required all of Field's splendid genius and energy to keep the project alive. In the face of repeated failures, and doubt as to whether messages could be sent rapidly enough to make any cable a commercial success, it was extremely difficult to raise fresh capital. America continued to evince interest in the cable, but with, the Civil War in progress it was not easy to raise funds. But no discouragement could deter Field. Though he suffered severely from seasickness, he crossed the Atlantic sixty-four times in behalf of the great enterprise which he had begun.