616 MISCELLANEOUS PAPERS. 



of the seventeenth century qnadrauts were employed for that purpose, 

 and so late as 1680 Flamsteed, the first English astronomer royal, thought 

 himself fortunate when he succeeded in construting one which enabled 

 him to be sure of his observed times within three seconds.* About 

 1690 Roemer invented the transit instrument, which soon superseded 

 the quadrant, and still remains the best appliance for determining time. 

 Most of his observations were destroyed by a fire in 1728, but the 

 few which have come down to us show that as early as 1706 he deter- 

 mined time with an accuracy which has not yet been very greatly sur- 

 passed. Probably the corrections found in the least square adjustment 

 of extensive systems of longitude determinations afford the best crite- 

 rion for estimating the accuracy of first-class modern time observations, 

 and from them it appears that the error of such observations may rise 

 as high as i 0.05 of a second. 



During the intervals between successive observations of the heav- 

 enly bodies we necessarily depend upon clocks and chronometers for 

 our knowledge of the time, and very erroneous ideas are frequently 

 entertained respecting the accuracy of their running. The subject is 

 one upon which it is difficult to obtain exact information, but there are 

 few time-pieces which will run for a week without varying more than 

 three-quarters of a second from their predicted error. As the number 

 of seconds in a week is 004,800, this amounts to saying that the best 

 time-pieces can be trusted to measure a week within one part in 756,000. 

 Nevertheless, clocks and chronometers are but adjuncts to our chief 

 time-piece, which is the earth itself, and upon the constancy of its ro- 

 tation depends the preservation of our present unit of time. Early in 

 this century Laplace and Poisson were believed to have proved that 

 the length of the sidereal day had not changed by so much as the one 

 hundredth part of a second during the last twenty-five hundred years, 

 but later investigations show that they were mistaken, and so far as 

 we can now see, the friction produced b;y' the tides in the ocean must be 

 steadily reducing the velocity with which the earth rotates about its 

 axis. The change is too slow to become sensible within the life-time of 

 a human being, but its ultimate consequences will be momentous. 



Ages ago it was remarked that all things run in cycles, and there is 

 enough truth in the saying to make it as applicable now as on the day 

 it was uttered. The Babylonian or Chaldean system of weights and 

 measures seems to be the original from which the Egyptian system was 

 derived, and is probably the most ancient of which we have any knowl- 

 edge. Its unit of length was the cubit, of which there were two varie- 

 ties, the natural and the royal. The foot was two-thirds of the natural 

 cubit. Eespecting the earliest Chaldean and Egyi)tian system of 

 weights no very satisfactory information exists, but the best authori- 

 ties agree that the weight of water contained in the measure of a cubic 



* Account of the Rev. Johu Flamsteed. By Francis Baily. pp. 45-'6. (London, 

 J835. 4to. pp. Ixxiv + 672.) 



