x PRACTICAL PURPOSE 269 



In the Navy it was found to be useless during gun-fire, 

 and to be affected so much by the rolling of the ships 

 during stormy weather that little dependence could be 

 placed upon it. These defects came directly under the 

 notice of Lord Kelvin after he had undertaken to write 

 an article for a magazine. ' When I tried," he said in 

 the article, " to write, on the mariner's compass, I found 

 I did not know nearly enough about it. So I had to 

 learn my subject. I have been learning it these five 

 years." He not only noted the defects of the existing 

 instrument, but also set himself to devise a means of 

 remedying them ; and in the end he produced the com- 

 pass which has made his name famous to every nautical 

 man. The existing compasses were made with needles 

 ten, twelve, or even fifteen inches in length, in order that, 

 when shaken, their period of vibration or swing should 

 be long. The card also was made large, with the view 

 of keeping the compass steady ; but the result was that, 

 on account of the weight of the needles and card upon 

 the pivot, the compass was apt to stick, and was always 

 sluggish in its action. 



Lord Kelvin first showed in a mathematical paper 

 that steadiness of the compass at sea in stormy weather 

 could be obtained by small needles and a light compass 

 card instead of large needles and a heavy card, if certain 

 conditions of construction were fulfilled. He took out 

 the first patent for his improvements in 1876, but he 

 had to wait thirteen years before the instrument was 

 adopted as the standard compass for the Navy. The 

 compass is now being supplemented by a new form in 

 which the magnets and card are immersed in a liquid 

 which fills the compass-bowl, and by the gyro-compass, 

 which is independent of magnetic conditions, but it was 

 Lord Kelvin who made the first departure from the crude 



