16 



lesser character, produce such remarkable phenomena may we 

 not hope that a better knowledge of the solar and planetary 

 influences will enable us to make some approach to meteorolo- 

 gical forecasts. 



By knowing the true distance of the sun from the earth, 

 and the greatest elongation of the planet, it becomes a simple 

 question in plane trigonometry to ascertain the distance and 

 size of all the planets in the system, a problem, the solu- 

 tion of which will for ever form an epoch in the history 

 of mankind. The complicated movements of our globe, 

 and the system to which it belongs, have been demon- 

 strated in a general proposition by Lagrange and Laplace, 

 viz., an invariable relation exists among the eccentricities 

 of any number of perturbed orbits, and that the sum of 

 the squares of the eccentricities, each multiplied by an 

 invariable co-efficient, is itself invariable, and subject to no 

 change by the mutual action of the parts of the system. The 

 masses of the planets, and the constants of their motion, might 

 all be changed from what they are (within certain limits) yet 

 the same tendency to self-destruction in the deviations of the 

 system (from a certain state) would still exist, so that at the 

 end of each period of the system, all its parts are re-established 

 in their original position to set out afresh, to run the same 

 unvarying round for ever. 



To James Gregory, a Scotch mathematician, is due the 

 original suggestion of using the inferior planets for getting the 

 parallax of the sun, although Halley has the credit of having 

 first recommended to the notice of future astronomers an 

 affecting exhortation not to suffer so precious an occasion 

 as the transit of Venus in 1631 to pass unprofitably, but to 

 deduce from this observation one of the most important 

 elements of our system. Halley at this time was aware of the 

 rarity of the transit of Venus, as the plane which Venus des- 

 cribes does not coincide with the plane of the earth's orbit 

 owing to which circumstance two transits usually, but not 

 always, occur in an interval of eight years, after which they 

 do not again occur for more than a century, and always in June 

 and December. If however a transit does not happen at the 

 same node after an interval of eight years, it cannot take place 

 again at that node for 235 years, which will be in the year 

 2117. 



Venus, seen from the earth, accomplishes an entire oscilla- 

 tion round the sun in 584 days, and then returns to inferior 

 conjunction again, but during this time the earth has made 

 one entire revolution round the sun, besides having described 

 an arc of about 216deg., five times which makes l,080deg., or 

 three circumferences of 360 deg., therefore at the end of five 



