CHK, 



CHR 



reign of Yezdejerd. The years of this sera, 

 like the Nabonassarean, consists of 12 

 months of 30 days, with an addition of 

 5 intercalary days at the end, making in 

 all 365 days. 



The limits of our plan will not allow us 

 to enter more minutely into the details of 

 this important science. For these we 

 must refer to separate treatise* on the 

 subject. The abstract which is here 

 given will, however, be found sufficient 

 for all the general purposes of the histo- 

 rical student. We have purposely re- 

 frained from giving a chronological table 

 of remarkable events, as such tables are 

 to be procured with very little trouble. 

 Various ingenious methods have been in- 

 vented, of associating the name of some 

 remarkable event with the date of its oc- 

 currence, with the view of impressing it 

 on the memory ; for some account of 

 these, we must refer to the article ME- 

 MORY ARTIFICIAL. 



CHRONOMETER, an instrument or 

 machine for measuring time. The word 

 is more particularly used by workmen 

 and navigators to denote a watch, or 

 portable machine, in which, by the na- 

 ture of the escapement and the com- 

 pensations for heat and cold, mean time 

 is or ought to be kept with sufficient 

 accuracy to determine the longitude at 

 sea. 



The relation between time and longi- 

 tude will be fully explained hereafter: it 

 will therefore be sufficient in this place 

 to remind the reader, that the rotation 

 of the earth upon its axis brings the seve- 

 ral places upon its surface, in succession, 

 oppos:tt, the sun, causing day and night ; 

 so -hat the absolute instant of noon, or 

 of any other determined apparent time 

 of the day, at eacli place must be earlier, 

 at a place which lies to the eastward 

 of another, with which that place may 

 be compared. From tin's general fact it 

 follows, that allowing 24 hours for the 

 whole rotation of the earth, and pro- 

 portionally for every smaller part of the 

 rotation, we may determine ^provided 

 the apparent time at two places be 

 known) what is the difference of longi- 

 tude between them- Thus, if a chrono- 

 meter set to the time at Greenwich were 

 to be carried to Petersburg!!, in Russia, 

 it would indicate time two hours laier 

 than the clocks at Greenwich ; that is to 

 say, it wouid shew when it was noon at 

 Greenwich, instead of when at Peters- 

 Imrg'h The obvious conclusion would 

 be, that the sun arrives at the meridian 

 of Petersburgh earlier, and consequently 



that this town lies more easterly than 

 Greenwich ; and as two hours are in pro- 

 portion to 24 hours, so is 360, the earth's 

 circumference, to 30, the longitude of 

 St. Petersburg, reckoned from Green- 

 wich. Upon the same principle it is, that 

 the clocks in a large town ought not to 

 indicate the*same time. Thus the clocks 

 at St. Paul's, St. Clement's, St. Martin's, 

 and St. George's, Hanover Square, in 

 London, ought to strike each four seconds 

 after the other ; and this difference, it 

 may be added, would nearly vanish, if 

 heard from any of the westerly stations, 

 on account of the time employed for the 

 passage of sound ; and for the same rea- 

 son it would be nearly doubled in the op- 

 posite direction. 



From the intimate relation which sub- 

 sists between the construction of watches 

 and clocks, the similitude of the escape- 

 ments, and the common principles upon 

 which the compensations for heat and 

 cold are effected in each, we shall ex- 

 plain the principles of each under the 

 general article HOROLOGE ; and at pre- 

 sent we shall only give an account of 

 the nature of the expedients adopted to 

 produce superior accuracy in these port- 

 able machines. 



The train of wheels, which constitutes 

 so large a part of every time-piece, must 

 necessarily transmit the force of the 

 first mover with periodical irregularities, 

 arising from oblique actions of their teeth 

 upon each other; and these irregulari- 

 ties will be subject to other variations, 

 arising from the greater or less degree of 

 fluidity in the oil applied to the pivots and 

 elsewhere. The first mover also in a 

 portable machine being a spring will be 

 more rigid, and consequently act with 

 greater power when cold than when hot. 

 The balance, or vibrating measurer of 

 the time, is a wheel, or equivalent piece, 

 fixed on an axis, upon which it could 

 freely turn ; but this liberty is restrained 

 by a fine spring", called the pendulum 

 spring, which is fastened to the axis, 

 and after taking several turns round 

 without touching' it, the other end of the 

 spring is fixed to the frame of the ma- 

 chine. By this contrivance the balance 

 will, if not prevented, come to rest in one 

 particular position ; and if at any time" 

 disturbed, it will only vibrate each way 

 from the line of quiescence, performing 

 larger or smaller arcs, according to the 

 disturbing force. This force in a watch 

 or time-keeper is communicated from the 

 train ; most commonly during the time oi 

 each vibration : and the machinery or 



