428 TRANSACTIONS OF SECTION E. 



in this matter is certainly remarkable. What would the ancient geographers 

 have said if they were told of the accuracy of a modern first-class triangulation, 

 such as that of our own Ordnance Survey or of the Survey of India ? 



Still absolute accuracy of measurement of any kind seems to be an impossi- 

 bility, and the best we can do, after all, is to approach it as near as we can, 

 and to arrange matters so that the inevitable errors will tend to balance one 

 another. Nature herself seems to object to perfection in measurement. For 

 instance, when we attempt to measure a distance, and have taken all precautions 

 we can, changes of temperature occur and alter the length of our measuring-tape, 

 and, in spite of all that has been done by manufacturing tapes of alloys of 

 different metals in order to counteract this effect, uncertainty must exist to some 

 extent. Then as regards our angular measuring instruments, not only must there 

 always be personal error and some imperfections in the graduation and centering, 

 but the change of temperature again comes in, affecting the metal, and attempting 

 to defeat our object of obtaining perfection. If we desire to measure the true 

 vertical angle, there is always the troublesome and uncertain effect of the refrac- 

 tion of the atmosphere, which makes the mountain-top appear in a different 

 place from where it really is, according to the heat, moisture in the air, and all 

 sorts of other unknown causes which, in spite of all the corrections we may 

 apply, occasion at least some uncertainty as to our result, whilst, in the case 

 of the sun or star, it is considerably worse. So great is this refraction that 

 when the sun appears to be just above the horizon, as you see it over the sea, 

 it is actually not there at all, and has gone down below the horizon. Of course 

 tables have been constructed to correct for all this, but no one can say that 

 they are really accurate, as the results depend so much upon local conditions, 

 and they must after all be considered merely devices for making the best of a 

 bad job. Then, again, when we have taken all possible care with the levelling 

 of a theodolite. Nature, through inequalities of gravity, has an unsuspected trick 

 of drawing the level out of its normal position, which introduces uncertainty, 

 and is often most bewildering in its result. But enough has been said on this 

 subject. The only safe rule for a surveyor to follow is never to assume that he 

 is correct, and to take his observations so that they tend to compensate one 

 another, whenever it is possible to do so. 



So far what I have said has had chiefly to do with some of the earlier 

 attempts at surveying and map-making, and the instruments and methods by 

 which these have been carried out ; and I will now try to give you an outline 

 of what lias been done in comparatively recent times, and state briefly the 

 present position of various parts of the world as regards the condition of their 

 mapping and the survey basis upon which their maps depend. 



Little by little civilised man, by his daring, his love of adventure, and the 

 necessities of events and circumstances, has penetrated into the unexplored parte 

 of the earth and pushed back the clouds and mists that so long shrouded them 

 from his knowledge, until at the present time the regions that are entirely un- 

 mapped are very few indeed, and do not amount to more than about one-seventh 

 of the whole land-surface of the globe, including the unexplored areas of the 

 Polar regions, which may be either land or water. Not content with a mere 

 vague acquaintance, he has striven for greater accuracy, and has turned to various 

 branches of science and called them to his aid, in order that he may obtain 

 more correct knowledge and a better comprehension of the earth's features. To 

 enable him to fix with definiteness the position of places upon its surface, map 

 out the various land-forms, and obtain their accurate measurements, he has 

 consulted the astronomer and mathematician. Commencing, as we have seen, 

 with the rudest instruments and measuring apparatus, these, as greater accuracy 

 was required, have gradually been improved, until the present-day appliances 

 and equipment of a surveyor are a wonder of refinement and delicacy. 



In order that we may obtain a general idea of what parts of the world have 

 been mapped and what have not, as well as ascertain something of the value 

 of the survey basis for maps of the various parts of the world at the present 

 time, I will now show a map I have recently drawn. It is merely an outline, 

 and diagrammatic in character ; but I trust will help to make the matter plain. 

 By way of comparison I have drawn another map showing what was surveyed 



