374 



Lieut.-Col. A. Strange on a new Theodolite. 



think that it is not judicious to apply the same method to a portable as to 

 a fixed instrument ; to do so introduces the difficulty of carriage, and this 

 must result in curtailing size, and with it power. Casting in masses pro- 

 fesses to diminish or eliminate the strains and tensions supposed to be in- 

 cident to building up the structure with separate pieces. Whatever may 

 have been the case some years ago, I am disposed to believe that this 

 superiority is now much less marked than it used to be, if, indeed, it exists 

 at all at present. Self-acting shaping-tools have been brought to such 

 perfection, that the fitting of contiguous surfaces to each other can now be 

 made practically absolute, to the exclusion of those strains which imperfect 

 fitting must of course formerly have introduced. Nor must it be forgotten 

 that even casting in masses does not exclude irregular strains, and that the 

 more unequal in thickness the different parts of a mass are, the more such 

 strains are to be apprehended. Let any one cast even so symmetrical a 

 thing as a ring, say 3 feet in diameter and 3 inches thick, divide it into two 

 halves, and afterwards attempt to fix these halves together ; he will find 

 them infallibly distorted. Every one who has turned large irregularly 

 shaped masses of metal in a very accurate lathe, knows that every cut of the 

 tool alters the general form of the mass. In the instrument under discus- 

 sion, the principle in question has, I now believe, been carried too far ; it 

 has greatly increased weight, whilst I doubt whether it has diminished 

 unequal tensions. 



As to choice of materials, I have already mentioned that the use of 

 aluminium bronze has tended materially to diminish weight. I am of 

 opinion, however, that steel can now be employed for such a purpose 

 almost exclusively, since at the present day it can be cast and worked as 

 readily as brass, though of course at rather greater expense, owing to its 

 hardness. The various shaping-machines and drill- slotting machines 

 give almost unlimited control over the forms that may be given to the 

 solid masses, while sheet-steel is now an article of ordinary commerce 

 available for the tubular parts. The nickelyzing process, now extensively 

 employed in America, and becoming every day better known and appre- 

 ciated here, effectually preserves steel from oxidation. 



Practical surveyors are alive to the question of facility in dismantling, 

 packing, and setting up so large an instrument as this. I am able to give 

 some data on this head. After exhibiting the instrument to the Society at 

 Burlington House, Piccadilly, it had to be removed to my Observatory in 

 Belvedere Road, Lambeth ; it was taken to pieces, packed, loaded on a 

 van, conveyed to its destination, and again set up in the Observatory 

 ready for adjustment and use in three hours. I should mention that the 

 passages through which the various parts had to be carried from the 

 Meeting-room to the cases in the outer hall are tortuous, narrow, and dim, 

 that the distance from Burlington House, about If mile, was traversed at 

 a foot's pace, in order that the men might be at hand in case of accidents, 

 and that ten ordinary labourers of the India Store Department, with my 

 assistant and myself, were employed in the operation. 



