132 PHILOSOPHICAL TRANSACTIONS. [aNNO 1704. 



performed as before directed. This, although in a manner the same with the 

 instrument before, yet is more convenient in some respects : chiefly because 

 the plumb lines may be made longer, and the sights set farther asunder, than 

 in the instrument before can conveniently be done; which is some little advan- 

 tage for seeing the transits. These sights may also be made so light, as to be 

 easily carried about ; or they may be readily made or imitated in any place 

 wherever you come. 



To know when the Polar Star comes on the Meridian. — The way is this: sub- 

 tract the right ascension of the sun from the right ascension of the pole-star, 

 the remainder gives the degrees, minutes, and seconds when the pole-star tran- 

 sits the meridian above the pole. Divide these degrees by 15, it gives the 

 hours, and every degree under 15 multiplied by 4, gives the minutes; and every 

 minute multiplied by 4, gives the seconds, of apparent time of the pole-star's 

 southing. I scarcely need to say that it comes under the pole at 12 hours 

 distance, only making some small allowance for the alteration of the sun's right 

 ascension in that 12 hours time. But you may shorten the labour, by using 

 tables of the sun's right ascension in time, instead of his right ascension in de- 

 grees, &c. which may be found in Sir John Moor's Math. Compend. and in 

 several other books. If the sun's right ascension exceed the pole star's, add 

 360 degrees, or 24 hours, and then subtract. 



The right ascension of the pole-star is determined by Mr. Flamsteed, to be 

 33m. 4s. of time. Anno 169O; and the increase of its right ascension in 

 10 years is Im. l6s. of time. Therefore this present year 1703, the right 

 ascension of the pole-star is 35m. 22s. of time. 



Or you may see when the pole-star comes to the meridian, by hanging up a 

 plumb-line, and observing when the thill-horse in Charles's Wain, called Alioth, 

 comes near the line, together with the pole- star, on one side the pole; or the 

 bright star of the 3d magnitude in Cassiopeia's thigh on the other side. 



The foregoing instruments may be set by any other star, as well as the pole- 

 star. But the pole-star, in our northern hemisphere, is most convenient, be- 

 cause it makes but a small circle round the pole, and therefore moves slower, 

 and consequently is longer in transiting the meridian. And therefore a small 

 error in calculation, or a little expence of time in setting the instrument,, may 

 be disregarded. 



The Uses of these Meridian Instruments. — 1. You may see with all imaginable 

 exactness, when it is noon, even to 1, 2, or at most 3 seconds of time. For 

 you may see when the very limb of the sun touches the meridian, and while all 

 his disk is passing it. So that it far exceeds all sun-dials. And besides another 

 great conveniency is, that it will fit most latitudes. So that there is no need 



