StPERIAL DAt. 49 



thought, that long and short vibrations will be performed in the 

 sam? time yet this is true, at least when the pendulum is quite 

 lonf, anU the arcs over which it swings are of moderate lengths. 

 Huygens conceived the idea of applying the pendulum to the 

 clock, as a regulator, and succeeded in accomplishing this, and 

 thus gave to the world an accurate measurer of time. The clock 

 thus perfected, became so accurate, that it was necessary to contrive 

 some more accurate means to regulate it. Hitherto, the successive 

 occultations of some star, observed without the aid of a telescope, 

 had been sufficient, and the time of noon, or 12 o'clock, was 

 obtained by sun-dials, and other means, with sufficient accuracy, 

 for the instruments hitherto employed. 



Any occurrence, which takes place at regular intervals, may be 

 adopted as a regulator of time, but the revolution of the earth on 

 its axis is by far the most accurate. For certain reasons, which 

 will be given presently, the sun is apparently subject to such 

 irregularities, that the solar days, or exact interval, from the' time 

 the sun is on the meridian, until his return to it again at the 

 successive revolution, are of unequal lengths. In other words, the 

 solar day is variable. Now the real revolution of the earth on its 

 axis, is the time in which any given meridian, or situation on the 

 earth, moves from a particular star, back to that star again. Thus : 

 A. 



Let A, B, O, D, be the earth, its north pole N, being towards 

 us, and suppose it revolving in the order of the letters. Let N D 

 be the meridian, or north and south line passing through some 

 particular spot, Greenwich, for example, shown at E, and let the 

 star S, be upon the meridian, that is, if this line was extended to 

 the heavens, or, more properly, a plane passing through this 



