19 



her mean value according to M. Arago, being 16-904". When 

 the elongation of Venus is 30° 44' between its inferior con- 

 junction and greatest elongation, it appears the brightest, and in 

 this situation, Venus is often seen with the unassisted eye in 

 broad daylight. At inferior conjunction the sun and Venus 

 approach each other at the rate of 4 sees, in the minute, so 

 that if the time of contact be erroneous at each place of ob- 

 servation 4 sees, of time, the angle may be erroneous, equal 

 to 8-15 of a second ; therefore the limit of error is about 4-15 

 of a second, so that an angle only 4-15 of a second can be 

 measured, a less quantity than can be determined with cer- 

 tainty by any other method. 



In conclusion : Suppose the two observers to be exactly 90° 

 apart, a circumstance scarcely to be expected,yet by knowing the 

 distance, and the greater the distance that the observers are 

 apart on the Earth's surface, the more correct is likely to be the 

 observation it will not be difficult to find the magnitude of 

 the chord-line within the Earth, and from hence deduce what 

 must have been the result had their distance been exactly 

 90°. 



From previous observations the parallax of the sun is included 

 within the limits of 8'5", and 8'7/' ; the mean 8'6'' has been 

 adopted by Delambra, and Laland, but is not considered 

 sufficiently correct. When the true distance of the sun has 

 been determined, and all the planets in the system adjusted 

 thereto, we may look forward with hope to the time when 

 the conservation of forces which unite the planetary to solar 

 influences may be brought to our knowledge, and give a 

 principle of abstraction which will enable observers to ob- 

 tain a correct investigation of those physical laws which 

 control the earth and its atmosphere. 



