220 IMPROVEMENT IN MB. B. WALKER's QUADRANT. 



Improvement ^i^f^^ i^^ a fmall mirror be fixed upon the top of the inde^ 



ker's r'efleaing g^afs. This re6tifier muft be placed parallel to the firft hori- 

 ^uadrant. zon glafs, when the index ftands at 90° on the arc, confe- 



quently the index glafs and redifier will form an angle of 45 

 degrees. Secondly, let the two horizon glalTes ftand as high 

 above the plane of the o6tant as the redlifier, and the inflru- 

 ment is ready for ufe. 



After the firft horizon glafs has been adjufted, bring the in- 

 dex to 90, and the reftifier will be parallel to the horizon glafs, 

 if the index glafs and reftifier form an exa6l angle of 90° on 

 the arc; but if they do not form that angle, the index will 

 iliow the error. Then to determine the error of the fecond 

 horizon glafs, let the index be brought to 0, and the reftifier 

 will in that fituation perform the fame office as the index glafs 

 in the quadrant, when the index fiands at 90 at N ; but as this 

 has been fully explained in the defcription of that inftrument, 

 it need not be further infilled on here. 



An odant of five inches radius, conftruded on thefe prin- 

 ciples, would be exceedingly portable, and fo flrong as not 

 to be eafily deranged by carriage; and thefe are properties 

 ■which may recommend it to the attention of the traveller by 

 land, particularly if he vifit thofe latitudes, where the altitude 

 of the fun fometimes exceeds 60 degrees. 



Thefe two glaffes may alfo be added to the fgxtant, without 

 depriving it of any of the valuable properties which it now 

 poflefTes: and this additional apparatus need only be ufed in 

 taking fuch angular diflances as are beyond the power of that 

 inftrument, 



I am, 

 SIR, 



Your's refpeclfully, 



E. WALKER. 



Lynn Regis, Odober 15th. 1803. 



Plate XI. Fig. 3. A B reprefents the index glafs; C D the 

 redifier; m the horizon glafs ; n the fecond horizon glafs. 



I 



1 



SCIENTIFIC 



