ch and such a volume of a certain standard substance鈥攖hat is, so many cubic inches, or parts of a cubic inch, of that substance. But what is an inch?31 It is determined, we find, as a certain fraction of the length of a pendulum vibrating seconds in the 鏉窞瀹跺涵spa latitude of London. A second, we know, is a certain portion of a mean solar day, and is practically determined by a reference to what is called a sidereal day鈥攖he interval, namely, between the successive passages by the same star of the celestial meridian of any fixed place. 鏉窞姘寸枟spa浼氭墍鍏ㄥ This interval is assumed to be constant, and it has, indeed, been described as the 鈥榦ne constant element鈥?known to astronomers.
We find, then, that there is a connection, and a very important connection, between the motion of the stars and our measures, not merely of value, but 鏉窞瑗挎箹鍘讳笉姝ｈ鎸夋懇 of weight, length, volume, and time. In fact, our whole system of weights and measures is founded on the apparent diurnal motion of
the sidereal system, that is, on the real diurnal rotation of the earth. We may look on the meridian-plane in which the great transit-telescope of the Greenwich 鏉窞涓嶆瑙剆pa鍦ㄥ摢 Observatory is made to swing, as the gigantic hand of a mighty dial, a hand which, extending outwards among the stars, traces out for us, by its motion among them, the exact progress of time, and so gives us the means of weighing, measuring, and valuing terrestrial 鏉窞姘寸枟浼氭墍鍏ㄥ objects with an exactitude which is at present beyond our wants.
The earth, then, is our 鈥榗hief time-piece,鈥?and it is of the correctness of this giant clock that I am now to speak.
But how can we test a time-piece whose motions32 we select to regulate every other time-piece? If a man 鏉窞姘寸枟浼氭墍398鑳藉共鍢?sets his watch every morning by the clock at Westminster, it is clearly impossible for him to test the accuracy of that clock by the motions of his watch. It would, indeed, be possible to detect any gross change of rate; but for the purpose of illustration I assume, what is indeed 鏉窞姘寸枟spa鍏荤敓 the case, 鏉窞姘寸枟浼氭墍楠岃瘉 that the clock is very accurate, and therefore that minute errors only are to be looked for even in long intervals of time. And just as the watch set by a clock cannot be made use of to test the clock for small errors, so our best time-pieces cannot be employed to 鏉窞鏈€濂界殑姘寸枟浼氭墍 detect slow variations, if any such exist, in the earth鈥檚 rotation-period.
Sir William Herschel, who early saw the importance of the subject, suggested another method. Some of the planets rotate in such a manner, and bear such distinct marks upon their surface, that it is possible, by a 鏉窞涓婇棬鍝佽尪 series of observations extending over a long interval of time, to determine the length of their rotation-period within a second or two. Supposing their rotation uniform, we at once obtain an accurate measure of time. Supposing their rotation not uniform, we obtain鈥?1) a hint 鏉窞涓嶆瑙勮冻娴村悕瀛?of the kind of change we are looking for; and (2), by the comparison of two or more planets, the means of guessing how the variation is to be distributed between the observed planets and our earth.
Unfortunately, it turned out that Jupiter, one of the planets from which Herschel 鏉窞鍝噷鎺ㄦ嬁濂?expected most, does33 not affordTags: 杭州养生按摩, 杭州品茶群上课, 杭州桑拿按摩一条龙, 杭州桑拿网丝足会所, 杭州足浴店按摩暗语, 滨江按摩不正规的