1872.] 



for the Indian Trigonometrical Survey. 



327 



Level Mountings. — The mode in which the levels are mounted was con- 

 trived by me, at the request of Sir Andrew Scott Waugh, then Surveyor- 

 General of India ; and I made the first example with my own hands, in 

 about 1853, for Troughton's Great Theodolite. The same arrangement has 

 since been applied to many other large instruments in India. Being rather 

 a matter of detail, I shall here only say that its object is to remove, as much 

 as possible, all restraint from the glass spirit-level, which should be allowed 

 to adapt itself to changes of temperature with perfect freedom. The level 

 is also encased in an external glass cylindrical cover, to protect it from 

 sudden currents of air tending to alter rapidly the temperature of the parts. 

 I believe that these principles were first applied by the Astronomer Royal 

 to the Greenwich Altazimuth ; but the details in my plan are somewhat 

 different, and, as I venture to think, more complete. The appliances for 

 adjusting the level in my plan are, I believe, new in their arrangement ; my 

 main object in devising them was to obviate strains, without introducing 

 risk of shake, and to improve delicacy of action. 



Material. — Whilst the instrument was under construction I became 

 acquainted, at the Great Exhibition of 1862, with aluminium bronze, an 

 alloy of 90 parts of copper with 10 parts of aluminium. Its properties 

 seemed to be exactly those required for the material of such an instrument. 

 With some difficulty, arising from the fact that no national establishment 

 exists in England for such purposes, I got some experiments made on the 

 alloy (partly by the makers, partly by the kindness of Mr. John Anderson, 

 C.E., of the Woolwich Gun Factories *), which showed that the rigidity of 

 the alloy might be taken at three times that of ordinary gun-metal. This 

 being the most important property for my purpose, I determined on using 

 it, and on reducing the thicknesses, not the depths, of all the lower parts 

 of my design. This would still leave such parts twice as rigid as if con- 

 structed of the previous dimensions in gun-metal. 



Accordingly the elevating screws of the stand and their bearings, the tri- 

 brach, horizontal circle, vertical-axis socket, horizontal-microscope arms, 

 and foundation-plate carrying the pillars are of aluminium bronze. The 

 remaining parts, having much less weight to bear, are of the usual 

 materials, gun-metal and yellow brass, which are more easily worked than 

 the bronze. 



Probable performance of the Instrument. — The trials I have as yet made 

 of the instrument lead to the conclusion that it is subject to no essential 

 defect, and that the objects sought in its construction have been to a great 

 extent attained. Actual work in the field, submitted to all the elaborate 

 verifications indispensable in modern geodesy, can, however, alone ascertain 

 its character ; and this must be a work of years. 



I now allude, as it may seem on first consideration rather prematurely, 



* See my paper, Monthly Notices of theR. Astron. Soc, 14th Nov. 1862, vol. xxiii. 

 No. 1, " On Aluminium Bronze as a Material for the Construction of Astronomical and 

 other Philosophical Instruments." 



