1865.] Astronomy. 447 



is also a subject of congratulation to find that, whatever communica- 

 tions may be made, there are members present who are able to add to 

 the value of the communications by their remarks, and who show that 

 they understand the subject fully, and are quite acquainted with its 

 progress. 



At the April Meeting of the Society, a paper "On an Aluminium 

 Bronze Transit Axis," and on " Certain New Methods of Adjusting 

 Transit Instruments," was brought forward by Lieut.-Colonel A. 

 Strange, F.E.S. Three years ago this gentleman presented to the 

 Society a paper advocating the use of aluminium bronze, as a substi- 

 tute for gun-metal in astronomical and surveying instruments. Since 

 that time the alloy has been used in the large instruments now being 

 made under his supervision for the great Trigonometrical Survey of 

 India. The experience gained during the construction of these shows 

 that the alloy is not fitted to receive graduation, both on account of 

 the metal tarnishing more than was at first expected, and owing to the 

 difficulty of obtaining absolute soundness of surface in casting of this 

 metal. At first also the use of the alloy was threatened with further 

 limitation. The early experiments in casting it in large masses 

 resulted in internal air-holes to an extent to which other metals are 

 less subject. This hindrance to the more extended use of the alloy 

 has, however, yielded to scientific effort, and in proof of this state- 

 ment there was exhibited to the Society the axis of a transit instru- 

 ment formed of aluminium bronze. It was 33£ inches between the bear- 

 ings ; the central cube is 9£ inches by the side, and the thickness of the 

 metal is only 0* 15 of an inch ; the whole weighing 56 1 lbs. This ad- 

 mirable specimen of scientific founding was from the works of Messrs. 

 Cooke, of Tork, who are one of the few optical firms (if not the only 

 one) in England who do the whole of their own founding. Having 

 regard to the great rigidity of the alloy (three times that of gun- 

 metal), and the ingenious system of internal strengthening webs, 

 there is little doubt that this is the stiffest transit axis in existence. 



In the second part of his paper, the author submitted to the 

 Society certain modifications which he proposed to introduce in the 

 two transit instruments, to one of which the axis exhibited belonged. 

 These came under the heads : I. Adjustments of axis for azimuth and 

 horizontality. II. Mode of applying the level to the axis. III. Mode 

 of examining the collimation error. This latter is intended to be 

 effected by means of a mercurial trough, as being a preferable arrange- 

 ment, for a travelling observatory, than by means of fixed collimators. 

 As it is difficult in a field observatory to prevent tremors and dis- 

 turbance of the mercury by currents of air, Mr. Cooke proposes the 

 following arrangement : — A disc of glass having plane and parallel 

 surfaces will float on the mercury and protect it from disturbance. 

 The lower surface of the disc, in contact with the mercury, will form 

 a brilliant plane-mirror to be used in the same manner as the usual 

 unprotected surface of mercury. Certain mechanical conditions are 

 imposed by the terms of the problem. 1st. The two surfaces of the 

 disc must be plane ; 2nd. They must be parallel ; 3rd. The, glass 



