IX 



" But the broad basis of observations upon which this magnificent 

 ^superstructure has been reared is British ; in the National Observa- 

 tory it was created. Such has been the mission of that establish- 

 ment, and such Mr. Airy has wisely judged it must continue to be, to 

 furnish now, and in all future time, in an unbroken series, the best 

 and most perfect data by which the laws of the lunar and planetary 

 movements, as developed by theory, can be compared with observa- 

 tion." 



In the report for 1841 he also describes how the Magnetic and 

 Meteorological Department had grown into an important branch of 

 the Observatory. He tells us that the regular work of the establish- 

 ment is to observe the meridional, bifilar, and horizontal needles, the 

 barometer and thermometers, besides several other instruments, every 

 two hours night and day, except on Sundays ; to pursue incessantly the 

 magnetic observations whenever anything unusual occurs ; to observe 

 some of the instruments every five minutes during twenty-four hours 

 on a fixed day every month. As the observations, when made, had 

 all to be reduced and tabulated in proper shape, it is clear that the 

 amount of work done must have been very great. Some of these 

 troublesome observations were afterwards abbreviated by a system of 

 self-registration by photography. The description of this new 

 system is given in the volume of ' Greenwich Observations ' for 1847. 



In April, 1839, Mr. Airy read to the Royal Society his first paper 

 on the correction of compasses. Captain Flinders, in his famous 

 voyage to Australia, had observed that the north end of his compass 

 appeared to be drawn towards the bows of his ship; and he, and 

 others after him, had suggested methods of compensating the cause of 

 the disturbance. The Astronomer Royal was, however, the first to 

 make a thorough investigation of the laws of magnetic disturbance. 

 The iron ship " Rainbow " was placed at his disposal with a view of 

 discovering by experiment some method of controlling the strange 

 deflections of the compass. The use that he made of this vessel will 

 make its name as famous in the history of the mariner's compass as 

 Stephenson's " Rocket " is in the history of locomotives. Assuming 

 that every particle of iron in the ship is, by the action of terrestrial 

 magnetism, converted into a magnet, he calculated the resolved 

 forces on one end of a compass needle whose centre holds a fixed 

 position in the ship. He found that these forces contained two sets 

 of terms, which he called the semi-circular and quadrantal variations, 

 their phases being respectively the azimuth and twice the azimuth of 

 the ship. The latter of these he found to be due to the induced 

 magnetism ; while in the former the permanent, the sub-permanent, 

 and the induced magnetism had shares. Having determined the 

 coefficients of these variations by observing the times of vibration of 

 a delicate needle placed first on shore and then on the ship, he was 



