LOCAL PABTICTJLAES OF THE TRANSIT OF VENrS, 1874. 73 



This is known as the direct method, aud will be one of those 

 used at the coming transit. But since the actual amount of dis- 

 placement of Venus is very small, all the measures must be made 

 with a faultless micrometer, and each observation really involves 

 two measures which should be made simaltaneously, viz., the 

 distance of Venus from the nearest part of the sun's edge, and its 

 angular distance from a meridian or north and south line, on 

 the sun. Both measures require extreme accuracy, and present 

 serious difficulties in actual operation. Eirst, the necessity to 

 make two measures at the same instant ; for since Venus is 

 moving in two directions, or at least appearing to do so, any 

 difference of time in the measures would destroy their value. 

 Another great difficulty is the ill-defined edge of the sun, which. 

 is constantly varying from atmospheric causes, and is never 

 sharply defined in any part, but liable to distortion or apparent 

 enlargement from various causes. 



It is possible, I think, that this difficulty might be overcome by 

 a micrometer made for the purpose, so that a pair of cross wires 

 being made to bisect the sun's limb in one part might be made to 

 follow it all round, and so by finding the average edge, measure 

 from the centre just as the solar photos are put on the plate of 

 the microscope and their average edge determined. But of course 

 such a method would require perfect clock-work. 



It will be observed that the direct method assumes an exact 

 knowledge of the dimensions of the earth, and the power of 

 finding exact positions, both of latitude and longitude, so as to 

 make allowance for difterence of time in the observation. Now, 

 a century since, the exact determination of longitude was not 

 possible, for the lunar theory Avas an unsolved problem, and the 

 lunar tables little better than guesses, upon which it was useless 

 to base any attempt to find accurate longitudes ; and it was well 

 known that if astronomers had to depend upon that method, 

 there was little chance of a satisfactory result. Hence the illus- 

 trious Halley was led to look for some other method, and devised 

 a way out of the difficulty. He saw that the relative lengths of 

 the paths of Venus, as observed from two stations, would give 

 data by which to measure the distance between o n and m g, or 

 the amount of displacement, without any need of determining 

 accurately the true positions of the observers. All that was 

 requisite was to choose stations so that the difference might be as 

 great as possible, and provide the observers with the means of 

 measuring the time of transit as accurately as possible. 



These conditions were easily fulfilled, but still observers went 

 to their work unprepared for the serious and perplexing difficulty 

 they had to meet. No sooner had the transit commenced and 

 the planet encroached more than half its diameter on the sun, 

 than it Avas found that its figure became distorted on the side 



