2 THE VOYAGE OF H.M.S. CHALLENGER. 



but one obstacle to a re-calculation of the Gaussian constants promising important 

 results — the necessity for a fresh magnetic survey of the regions south of the parallel 

 of 40°. The observations made by the memorable Antarctic Expedition under Ross in 

 1839-43 were of immense importance when taken in connection with those made in 

 other portions of the world about the same date, and at different epochs where the 

 secular change was known ; but enough has since been ascertained to show that con- 

 siderable changes have been going on in Antarctic regions, and until these changes are 

 accurately known by means of extended observation the data for calculation must 

 remain imperfect. 



In 1868 Sir Edward Sabine read No. XL of his "Contributions to Magnetism" 

 before the Royal Society, being the first of four papers on the Magnetic Survey of the 

 Globe for the epoch 1842-45, the last of these being read in June 1876, just as the 

 Challenger had completed her voyage. As Sabine's maps accompanying these con- 

 tributions serve as a point of departure with which subsequent maps may be compared, 

 it seems proper to recall here some of the details of the observations upon which they 

 were founded. 



From all that is now known of the secular change of the magnetic elements, the 

 mean epoch 1842-45 was wisely chosen by Sabine for his magnetic survey. As already 

 remarked, it was about this time that the magnetic survey of the Antarctic region was 

 undertaken by Ross, and others subsequently completed much that he was unable to 

 do, and therefore the question of correction for secular change might, without serious 

 error, be neglected for that part of the world. But in the Arctic regions and temperate 

 zones observations had been so multiplied for different epochs that those made several 

 years before and after the mean epoch could be reduced thereto by the known secular 

 change, and therefore utilized. A glance at Sabine's maps shows that for Europe he 

 was well supplied with data for his lines of equal value, in North and South America 

 and some parts of Asia fairly so, whilst Africa presents almost a blank as regards 

 inclination and intensity, although the collected observations range over the years 

 1818-71. These lines give normal values; for Sabine, knowing full well the uncertain 

 distribution of local magnetic disturbance on land, always placed a high value on sea 

 observations. He found observers had done ample work for the North and South 

 Atlantic Oceans, and in a less degree in other seas except the North and South Pacific 

 Oceans, for which his maps are almost blank, the lines being only given for certain j)arts. 



One object in recalling these facts is to show that, valuable as is Sabine's Magnetic 

 Survey of the Globe as the first of its kind in which the intensity is included for so 

 large a portion of its navigable seas as well as the land, the whole forming a standard 

 of comparison for succeeding surveys, there remained a large field for observations in 

 parts of the world hitherto unvisited for magnetic purposes ; and further, to show 

 how the Challenger Expedition not only filled up these gaps but added largely to our 



