[ 537 ] 
both equally neceffary in their turn to be ufed by 
the pradical Aftronomer, and confequently to have 
a place in every well-furniflied Obfervatory. Far be 
it from me to fay any thing to the difparagement of 
either of thefe valuable indruments, or to envy it 
tlie place which it is fo judly entitled to. Every 
Adronomer, who has time and inclination for making 
a variety of obfervations, would undoubtedly wifh to 
be fupplied with, and to make ufe of both. But, 
as every perfon defirous of making obfervations for 
his own amufement or public utility may not hap- 
pen aftually to be furnidied with, nor chufe to be 
at the expence of providing himfelf with both, it 
is certainly a very defirable thing, if he could be 
enabled to make that ufe of the indrument he has, 
which might fupply, in fome meafure at lead, the 
want of the other which he has not. Therefore, as 
I find that the objedt-glafs micrometer may be ap- 
plied with little trouble and but fmall additional 
expence to the meafuring diderences of right afeen- 
fion and declination, with an exadtnefs little, if at all,' 
inferior to what they can be obtained with the com- 
mon micrometer, I propofe to give here the direc- 
tions necedary to be followed when it is ufed in this 
manner. I diall afterwards diew how differences of 
right afeenfion and declination between the limbs of 
the Sun and Venus or Mercury, and didances of the 
limbs both in lines parallel and perpendicular to the 
equator, may alfo be obferved in the tranfits of thefe 
planets over the Sun. Examples of the fecond and 
third of thefe methods may be feen in the obferva- 
tions of the late tranfit of Venus at the North Cape, 
and in the South Seas, made according to thefe di- 
VoL. LXI. Z z z redlions, 
