VOL. XXIX.] PHILOSOPHICAL TRANSACTIONS. 245 



and that without any other instruments than telescopes and good common 

 clocks, and without any other qualifications in the observer than fidelity and 

 diligence, with a little skill in astronomy. For we need not be scrupulous in 

 finding the latitude of the place, or in accurately determining the hours with 

 respect to the meridian ; it is sufBcient, if the times be reckoned by clocks, 

 truly corrected according to the revolutions of the heavens, from the total 

 ingress of Venus on the sun's disk, to the beginning of her egress from it, 

 when her opaque globe begins to touch the bright limb of the sun ; which 

 times, as I found by experience, may be observed even to a single second 

 of time. 



But by the limited laws of motion, Venus is very rarely seen within the sun's 

 disk; and for a series of 120 years, and upwards, is not to be seen there once; 

 viz. from 1 639, when Mr. Horrox was favoured with this agreeable sight, and 

 he the first and only one since the creation of the world, down to 176I; at 

 which time, according to the theories hitherto observed in the heavens, Venus 

 will pass over the sun on May 26 in the morning; so that (vide Phil. Trans. 

 N° 1 93) at London, nearly at 6 o'clock in the morning, she is to be in the 

 middle of the sun's disk, and but 4 minutes more southerly than his centre. 

 The duration of this transit will be almost 8 hours; viz. from 2 till near 10 

 o'clock in the morning. Consequently her ingress will not be visible in 

 England: but the sun at that time being in l6° of Gemini, and almost in 23° 

 of north declination, will be seen not to set throughout the whole northern 

 frigid zone ; and consequently the inhabitants of the coast of Norway, as far 

 as its northern promontory, beyond the town of Drontheim, may observe 

 Venus entering the sun's disk ; and perhaps this ingress into the sun at his 

 rising may be seen by the inhabitants of the north of Scotland and those of 

 Zetland. But when Venus is nearest the sun's centre, he will be vertical to the 

 northern coasts of the gulph of Ganga, or rather of the kingdom of Pegu ; 

 and consequently, in the neighbouring countries, when the sun shall, at the 

 ingress of Venus, be almost 4 hours distant to the east, and almost as many 

 to the west at her egress, her apparent motion within the sun's disk will be 

 accelerated almost twice as much as in the horizontal parallax of Venus from 

 the sun; because Venus at that time moves retrograde from east to west; while 

 in the mean time an eye, on the surface of the earth, is carried the contrary 

 way, from west to east. 



Supposing the sun's parallax, as was said, to be 1 2 seconds and a half, Venus's 

 parallax will be 43 seconds ; and subtracting the sun's parallax, there will remain 

 half a minute at least for the horizontal parallax of Venus from the sun, and 

 consequently, Venus's motion will be accelerated f of a minute at least by that 



