544 LECTURE XLV. 



to the time of our first meridian, and then, by comparison with the usual 

 determinations of time in any other place, to which they might be carried, 

 the longitude of this place might be found with perfect accuracy. Such 

 timekeepers as we have are indeed sufficiently correct, to be of considerable 

 utility, but it is necessary to compare them frequently with astronomical 

 observations of phenomena, which occur at times capable of a correct calcu- 

 lation. Sometimes the transits of Mercury and Venus, or the eclipses of 

 the moon, are employed for this purpose, but more usually the eclipses of the 

 satellites of Jupiter; these, however, cannot be well observed without a 

 more powerful telescope than can be employed at sea; and the theory of the 

 moon's motion, has of late years been so much improved, that her distance 

 from the sun or from a fixed star can be calculated, with sufficient accuracy, 

 for determining the time in London or at Paris without an error of one third 

 of a minute; so that supposing the observation could be rendered perfectly 

 correct, the longitude might be thus ascertained within aboutone twelfth of 

 a degree, or at most five nautical miles. 



The observed parallax of the sun and moon may be employed for the 

 determination of their distances from the earth ; but in the ease of the sun. the 

 simple comparison of his calculated with his apparent altitude is insufficient for 

 ascertaining the magnitude of the parallax with accuracy. Sometimes the 

 parallax of Mars, which is considerably greater than the sun's, has been 

 directly measured; but the most correct mode of ascertaining the actual 

 dimensions of the solar system is, to observe a transit of Venus over the sun's 

 disc, at two places situated in opposite parts of the earth's surface. For, 

 since the diurnal motion of some parts of the earth is directed the same way 

 Avith the motion of Venus iu her orbit, and that of others the contrary way, 

 the different effects of these motions must furnish a mode of comparing the 

 rotatory velocity of the earth, with the progressive velocity of Venus, and 

 consequently of inferring, from the known velocity with which the earth's 

 surface revolves, the actual velocity of Venus, and her distance from the 

 sun; whence the distances of all the other planets may be readily deduced. 

 (Plate XXXV. Fig. 514.) 



Our countryman Horrox was the first that particularly attended to the phe- 

 nomena of a transit of Venus over the sun's disc: Dr. Halley, when be 



