Diminution of the OHiqitiiy of the Ecliptic. 425 



coincidence in the actual state of the ancient observations, 

 and the deductions drawn from theory, when we possess, as 

 we hope soon to do, the means of settling this epoeha and 

 actual state of diminution at the present period. The grand 

 mural circle now making for the Royal Observatory, by 

 Mr. Troughlon, will, it is expected, be in readiness for ob- 

 servation early in the ensuing year; and we cannot doubt 

 but that the first object to be determined with it, will be 

 the settling of the above-mentioned data, so essential in the 

 theory and practice of astronomy. TLe observations of 

 Mr. Pond have clearly tended to show errors in the mural 

 quadrants at Greenwich of considerable magnitude ; and 

 whatever accuracy these instruments might originally possess, 

 we have now no hesitation, from a comparison of contem- 

 porary observations, not only of Mr. Pond, but of Mr. 

 Groombridge, Dr. Brinklv, &c, to express our satisfaction 

 on this point. Indeed it is not difficult to account for the 

 above-mentioned errors, when we take into consideration 

 the construction of the mural quadrants, and of the obser- 

 vatory in which they are contained. These instruments are 

 freely suspended, in a line nearly with their centre of gra- 

 vity, by two pins from a large stone pier; the upper and 

 'lower parts are therefore very differently affected by every 

 change of temperature in the atmosphere: for instance, if 

 we suppose an increase of heat to be equally diffused over 

 the whole instrument; the upper part (for "we must consi- 

 der the expansion as taking place from the point of suspen- 

 sion) will be less affected than the lower, it. having the ac- 

 tion of gravity counteracting the force of expansion, whilst 

 the lower part is assisted in its expansion from the same 

 force of gravity, and vice versa ; or, in other words_, with its 

 own weight, which in these instruments is very great, the 

 whole I think amounting to nearly 1000 pounds each. We 

 shall therefore observe, that it is scarcely possible for these 

 instruments to have retained their original accuracy for but 

 a very short space of time after they were first put up : the 

 change however might be inperceptible for several years ; 

 but as we now know that metals when continually acted 

 upon, do not retain their first figure except where the parts 

 arc duly balanced, we can easilvsee how the change of figure 

 must necessarily happen. In running our eye over the ob- 

 servations made by Dr. Bradley with Bird's mural quadrant, 

 we find frequent mention, when observing the stars in the 

 night-time, and particularly when the difference in the tem- 

 perature of the atmosphere within and without was very con- 

 siderable, 



