172 



Anniversary Meeting, 



[Nov. 30, 



niiralty ; and though made more fit for use in the field by improvements 

 suggested by the Astronomer Royal, the transport of an instrument at once 

 ponderous and delicate, through a wild and rugged country, was an under- 

 taking of no ordinary difficulty ; but it was performed without injury. 

 The terrestrial angles were taken with a 20-inch circle by Jones, and a 

 smaller theodolite by Reichenbach, both of remarkable precision. The 

 base, from which all the distances were derived, was measured with the 

 compensation bars used in the Irish Triangulation. Thus, in respect to 

 the means employed, this arc of the meridian may be regarded as inferior 

 to none on record. A. full account of the whole was completed in 1866, 

 and has been published by the Admiralty in two quarto volumes. It does 

 not confirm the abnormal value obtained by La Caille, but shows a probable 

 cause for the discordance. La Caille's northern station was in a hollow 

 surrounded by mountains, one of which, half a mile distant to the north, 

 was a mass of rock 2000 feet high, and others, at distances somewhat 

 greater, were still near enough to create disturbance. A station so situated 

 was obviously ill suited to be a terminal station ; and the triangulation was 

 extended across an immense plain of sand (the Bushman's Flat) to a point 

 without any visible source of local attraction. By this extension, and by a 

 similar one to the south, Maclear's arc has an amplitude nearly four times 

 as great as that of La Caille, and is on this account, as well as on account 

 of the greater accuracy in detail, far more deserving of confidence. The 

 degree which is derived from it is 1 133 feet shorter than that of La Caille ; 

 and as La Caille's is 1051 feet longer than that given by the spheroid, 

 which, according to Airy, represents the average of northern arcs, it is evi- 

 dently a near approximation to the truth. This is even more distinctly shown 

 by the close agreement of the latitudes computed from the geodetic mea- 

 surements with those given by the sector— that of the north extremity 

 being 0"'4 in defect, that of the south extremity 0"'5 in excess. 



The Philosophical Transactions of the past year contain an important 

 memoir by Mr. Abel, F.R.S., to which has been assigned the distinction of 

 forming the Bakerian Lecture for the year. It is a most careful and 

 exhaustive treatise upon the circumstances which infiuence the lability of 

 gun-cotton. He has made numerous experiments, both in the laboratory on 

 small quantities, and in store upon large quantities, of the material ; and 

 some of his experiments have been carried on upon the same sample for 

 three or four years. The results arrived at in these investigations show 

 that gun-cotton, purified according to Von Lenk's directions, may be kept 

 either in the open air or in closed vessels, and may be exposed to diffused 

 daylight for very long periods, without undergoing any change. The pre- 

 servation of the material for between three and four years has been perfect.. 

 By prolonged exposure to sunlight, ordinary gun-cotton suffers a gradual 

 decomposition, which is somewhat more rapid when the cotton is damp 

 than when it is dry ; but, even under these circumstances, the change pro- 

 duced by several months of exposure is of a very trifling nature, and may 



