726 SEC. 15. GEOGRAPHY. 



Two base lines have been measured with this apparatus, one in the north 

 of Ireland, in 1827, and the other on Salisbury Plain, in 1849. Upon these 

 two lines depend all the results of the trigonometrical survey. 



2944. Ramsden's Theodolite. 



This instrument was made by the celebrated Ranisden, and first used in 

 1792. Since that time it has been in use at a vast number of stations, in- 

 cluding the highest mountains in England and Scotland, the Hebrides, 

 Orkney, and Shetland. It is now in as good condition as when it was first 

 made, having never in all its travels met with any accident or ill-usage. 

 Subsequent to the year 1843, the observations with it have been made by 

 non-commissioned officers of the Royal Engineers. This instrument is a 

 slight improvement on another made by Ramsden, the .'3 -ft. theodolite of 

 the Royal Society, which was completed and first used in July 1787, but 

 the two are almost identical* The divisions of the circle on the Ordnance 

 Theodolite are rather closer than in the Royal Society's. The focal length 

 of the telescope is 36 inches, and the aperture 2 5 inches, the magnifying 

 power ordinarily used 54. The probable error of a single observation of a 

 fine object under favourable circumstances is about 0' 18". The astronomical 

 determination of the direction of the meridian has been effected at many 

 stations with this instrument. In 1862 it was used at Fairlight, near 

 Hastings, and St. Peter's Church, Isle of Thanet, in the extension of the 

 English Triangulation into France and Belgium. 



Many of the observations made with this theodolite have been to points 

 upwards of 100 miles off (seen by means of a heliostat, a looking-glass reflect- 

 ing the sun's light). The greatest distance ever observed was 120 miles, 

 from Ben More in South Uist to a hill in Sutherlandshire. 



The diameter of the circle of this instrument is 36 inches. The weight of 

 the whole is 200 Ibs. 



2945. Ramsden 9 s Small Theodolite. 



This instrument is very similar in appearance and construction to the 

 large instrument, but is only 18 inches in diameter. The focal length of the 

 telescope is 19 '5 inches, the aperture of the object glass 2 inches, and the 

 ordinarily used magnifying power 30. The horizontal circle is read by three 

 microscopes. 



It has been used at a very large number of stations, many of them being 

 church towers and spires. At Thaxted Church, of which the tower is 79 feet 

 high, surmounted by a spire of 93 feet, the instrument was 1 78 feet above 

 the ground. It was also used at St. Paul's Cathedral, over the cross. At 

 one of the stations in the north of France, some 14 miles east of Boulogne, this 

 theodolite was mounted on a scaffolding 75 feet high. 



2946. Troughton and Simms' 24-Inch Theodolite. 



This theodolite, of a totally different construction from Ramsden's, was 

 made by Messrs. Troughton and Simms, at the commencement of the Irish 

 Survey. In Ramsden's theodolites, the divided horizontal circle revolves with 

 the telescope, while the microscopes which read it are fixtures. In Simms', 

 the microscopes are connected with the telescope, and rotate with it, while 

 the circle is fixed. 



The focal length of the telescope is 27 inches, and the aperture 2 125 inches. 

 The instrument has a repeating table, and is, strictly speaking, an altazimuth. 

 The horizontal circle is read by five micrometer microscopes. 



It has been in continual use, at a very large number of stations, from 1829 

 until 1862, and, like Ramsdeu's theodolites, has been so fortunate as to escape 

 accidents. 



