Royal Institution of Great Britain, 177 



To take horizontal or oblique anj^les, the vertical column is un- 

 screwed from the stand, and the ball and socket joint screwed in 

 its place. 



With this instrument the time may be deduced from the sun's 

 altitude, taken under fiivourable circumstances, to within tjiree- 

 lenths of a second of the truth ; and the latitude deduced from a 

 single observation of the pole-star will seldom differ more than 

 twenty seconds from the truth, or, by a mean of seven or eight 

 observations, it may be determined to five seconds. 



The three horizontal threads with which the telescope is fur- 

 nished make it a very efficient equal altitude instrument. 



Tlie great advantages of this instrument over a sextant, to 

 persons travelling on land, are the facility and expedition with 

 which it can be used, requiring none of those troublesome adjust- 

 ments which in all reflecting instruments are necessary ; being a 

 perfect circle, with two verniers, any errors of eccentricity are cor- 

 rected ; it can be used when the sun or star is in the zenith^ which 

 gives it an immense advantage in tropical latitudes over the 

 sextant, and it renders unnecessary that troublesome auxiliary, an 

 artificial horizon. If the sun's altitude be taken, and a different 

 limb be brought to the horizontal wire, when observing with the 

 instrument turned half round, the mean will give the apparent 

 altitude of the sun's centre, consequently there will be no allowance 

 necessary for semi-diameter. 



All the parts essential to accuracy are finished in the best manner; 

 and those parts, where a high finish would only add to the expense, 

 are left in a rough state, and painted. The whole is packed in a 

 mahogany box, seven inches long, by four wide, and three deep, 

 •which also contains a zenith eye-piece. Mr. Robinson, of Devon- 

 shire-street, Portland-place, is the maker, and he charges seven 

 guineas. 



This instrument, when intended for more accurate surveys, has 

 a horizontal as well as a vertical circle. The horizontal circle is 

 furnished with three equidistant verniers, and a lower telescope, 

 which, when directed to a fixed object, indicates any accidental 

 derangement which might take place in the position of the instru- 

 ment. The price of the instrument thus constructed is ten guineas. 



Numerous presents were upon the tables, and amongst the rest, 

 some very fine sjyecimens of crystallized glass, from Isaac Cook- 

 son, jun., Esq., of Newcastle. 



JAN.— MARCH, 1830. N 



