370 METHOD OF EXAMINING ASTRONOMICAL INSTRUMENTS. 



the instrument, totally unnecessary. Whoever indeed era- 

 ploys this method of examination will be virtually the di- 

 vider of his own instrument; and all that he will ask of the 

 artist is, to make him a point about the end of every five or 

 ten minutes, the distance of which from zero he will deter- 

 mine for himself, and enter in his book to be referred to 

 counteracts bad when wanted. We may likewise observe, that by this ex- 

 centring and amination we shall not only be secured aarainst the errours 

 im perfection of # J ° 



figurej of division, but against those which arise from bad centring, 



and from the imperfect figure of the circle, and which in 

 general are of too great a magnitude to be neglected, 

 may be em- It will, I dare say, have occurred to you, that, whenever 



ployed to ob- wc are d cs j rous that an observation should be particularly 

 ▼iate errours * * 



from inequality exact, we may guard it against the effects of unequal ex- 

 of teinperature ; p ans j on or con t ra ction in the metal, by means of the ap- 

 paratus which I have described : for we have only to measure 

 the arc which has been determined by the observation against 

 the whole circle, or against the multiple of it, which ap- 

 proaches nearest to the circle, and thence to deduce its 

 value in the manner explained above; and we shall either 

 have entirely excluded the errour which we apprehended, 

 or have rendered it too small to be of any importance. 

 Suppose, for instance, that the arc determined by the ob- 

 servation was 48° ; then by measuring it against the whole 

 circumference increased by an arc of 24°, we shall obtain a 

 result free from any greater errour of unequal temperature, 

 , than one eighth of the increase or decrease of this arc of 

 24° beyond a due proportion to that of the circle itself, 

 gives all the ad- This expedient gives us all the advantages of the French 



vantages of t ie c j rc j e f repetition, without the inconvenience arising.from 

 French circle of * ' & 



repetition with, being obliged to turn the instrument, and move the teles- 

 out its inron- CO p e so many times in the course of the observation. Nay, 

 venieuces, and *• ? J •" 



is even superior I am persuaded, that the result may be made more accurate 



wn > in this way, than by the French method; because not only 



can the object be more frequently observed, but the con- 

 tacts or bisections, it may be presumed, will be more exact, 

 when the observer is not disturbed by the hurry attendant 

 upon the use of the repeating circle; and with respect to 

 any errour in the instrument, from whatever cause it may 

 arise, it will be as effectually excluded by the process which 



Ire- 



