420 Memoir on the Diminution of the 



siderahle, that it was necessary to lower the quadrant a little ; 

 and it was a practice always to keep the plumb-line constantly 

 over the point at bottom, without any regard to the variable 

 temperature of the instrument. Admitting, therefore, the in- 

 strument at the commencement of observation to have had an 

 equable temperature and to have been nicely adjusted; it is 

 evident that a readjustment, after a partial effect has taken 

 place, is to throw so much error into the observation. I have 

 often been surprised that so sagacious a man as Dr. Bradley 

 should not have noticed this circumstance. It accounts* in 

 the most satisfactory manner for, the disagreement in the 

 deductions of the observation of the stars in the feet of Ge- 

 mini, used in carrying forward the error of collimation de- 

 duced from comparative observations with the zenith sector 

 and quadrant. The variations in these instruments are 

 likewise rendered still greater from the construct ion of the 

 Observatory, which, having a slanting roof, receives the 

 Fun's rays almost perpendicularly upon it, and is so much 

 heated in summer as to occasion a change of temperature 

 in the top and bottom of the room amounting to 10° or 

 1-2°. Such a roof is perhaps capable of many facilities in 

 the opening and shutting the necessary shutters, but I think 

 it is the very worst form for an observatory. The adoption 

 of circular instruments in the &tead of quadrants is one of 

 the greatest improvements in practical astronomy. In the 

 instrument now making by Mr. Trough ton, any part may 

 be made the depending one, and the whole instrument be- 

 ing constantly turned about must preserve its proper figure. 

 The article which I have herewith sent you, sir, being a 

 very long one, I will not trespass further at present, intend- 

 ing at some future period to take an extensive view of the 

 progress and im proved state of practical astronomy* 

 Your very obliged 



and obedient humble servant, 



Thomas Firmingkr. 



Although the successive diminution of the obliquity of 



the ecliptic, as we approach to modem times, cannot now 

 be controverted, still it is with the greatest interest that 

 we witness at the end of ages the slow development of 

 the great inequalities of the system of the world. In 

 aiter ages, when with the results of theory a long series of 

 very accurate observations can be compared, this sublime 

 spectacle can be enjoyed much better than it is by us, to 

 whom antiquity has only transmitted such observations a,s 



oftea*« 



