VOL. XXI >^.] PHILOSOPHICAL TRANSACTIONS. 133 



of having a strict regard to the elevation of the pole, nor any danger of error 



, in making and setting, as is in most other instruments; but all is with ease and 



certainty performed. Therefore, 2dly, Into whatever place you come, you may 



easily see the errors of the sun dials there; and which go truest, and which false* 



3dly, As the sun, so also the fixed stars may be seen to transit (he meridian ; 

 by which the hour of the night may as exactly be known, as of the day by the 

 sun, knowing the right ascension of the star that transits. For, as above, 

 subtract the right ascension of the sun from the right ascension of the star, 

 the remainder converted into time, is the time of that star's culmination or 

 southing. And if \1 hours be added or subtracted, making due allowance for 

 the alteration of the sun's right ascension in that time, it shows the exact time 

 of that star's coming to the meridian northward. 



4thly, The hour of the day and night being thus, to 1, 2, or 3 seconds dis- 

 coverable, by the aforesaid instruments, I doubt not but that they may be use- 

 ful in finding the exact differences of meridians, either by the eclipses of 

 Jupiter's satellites, or the occultation of the fixed stars by the moon. 



I do not pretend that these instruments are any otherwise useful in finding 

 the longitude, than by showing the exact time of the day or night ; which is 

 one thing absolutely necessary in this matter. Neither will they serve without a 

 well adjusted pendulum-watch, or pocket- watch, that will keep time exactly 

 from one observation by the meridian-instrument to another. Nor are they 

 useful on shipboard ; but only on land, where they may remain fixed. Though 

 on head-lands, or any where on shore, they may be useful to the seaman. And 

 indeed, till better discoveries are made, they may be of service wherever long 

 telescopes can be used, for seeing the appulses of the moon to the fixed stars, 

 or the eclipses of Jupiter's satellites ; which is only on land. Unless a conveni- 

 ent standing for a man, and a telescope might be hung pendulously in a ship, 

 which, especially in a calm sea, may be as little subject to disturbance, as the 

 pendulums of watches are, which will retain their motion at sea. 



5thly, You may with all exactness continue a meridian line for many miles, 

 by looking through either sight, and observing what objects are intersected by 

 the plumb-lines. 



6thly, These instruments are prepared with little cost or trouble, and easily 

 carried about, or imrtable in any place, the latter especially. 



^ further account of the Pediculus Pulsatortus, or Death- fFatch,*' continued 

 from N° 271 . % the Rev. Mr. JVm. Derham, t. R. S. N** 291, p, 1586. 

 Plate 3, fig. 23 shows the death-watch, as it appears to the naked eye, and 



* The insect here described, is the termes pulsatorium. Linn. 



