geodetic instruments, as these inquiries would warrant us to be. There are certain 

 advantages derived from the use of the lighter aluminum instead of copper and its 

 alloys, the metals now employed for field-Instruments; but the disadvantages are 

 that pure aluminum, although very rigid, is nevertheless a very soft metal lik 

 and that, when alloyed with 10 per cent, copper, to make it harder, it becomes very 

 brittle, but when alloyed with 20 per cent, or 30 per cent, of copper, it becomes so 

 1 >rittle as to break like glass. Therefore, we believe, in the present state of its 

 development it is not a suitable material for precision instruments. 



An alloy of 95 parts aluminum and 5 parts of silver by weight has been found to 

 give good results, being more rigid and harder than the pun- meia!, and but little 

 heavier, while it is almost as resistent to corrosion, polishes well, and is said t<> 

 be better for graduation; but, the fact that it contains silver, will, of necessity, 

 limit its use to the more exceptional class of work. 



Very little is gained in the way of reducing the weight of an instrument by em- 

 ploying aluminum bronze (90 per cent, copper and 10 per cent, aluminum). The 

 parts of instruments made of the latter metal might be easily reduced somewhat 

 in thickness on account of its greater rigidity as compared with copper alloys ; yet 

 to lessen the tendency to vibration, and also in order to withstand the wear and 

 tear of the field use of an instrument, such parts need a little more mass, or dead 

 weight as it may be called. It is then found that the weight of an instrument re- 

 mains materially the same as ever. An exception to the rule may exist in the con- 

 struction of the larger and stationary astronomical instruments, whore aluminum 

 bronze may be used to a certain extent to advantage. Its adoption is, however, 

 restricted to non-revolving parts, since, when closely fitted into bearings made of 

 the softer copper and tin alloys, the friction and wear of these parts is so marked 

 lhat we would never think of substituting it for steel, bell metal or phospho~ 

 bronze, or for any work requiring a smooth and accurate motion. 



There can be no doubt that aluminum possesses great utility over brass in the 

 construction of instruments of minor importance. Sextants, reflecting circles, and 

 Ihe more ordinary compasses,* parts of plane-tables, etc., can be made of it with 

 propriety. We have used it occasionally for many years, but for reasons already 

 stated above, we are not prepared to advocate its general adoption for instruments 

 requiring greater precision, such as the finer transits, theodolites, etc. It is only 

 ia rare cases when a judicious use of this metal may be a necessity for the success- 

 ful construction of an instrument, as for instance in our new style of mining transit, 

 permitting of vertical sights up and down a shaft without the use of an extra side 

 1.3lescope, where certain detachable parts of the instruments are mounted in an ex- 

 centric position, and unless such parts are made of aluminum they would require a 

 heavy counterpoise. 



It is principally the indiscriminate use of aluminum that we would warn against. 

 We are aware that transits have been made of aluminum, but aside from their nov- 

 elty as such, little or no merit can be claimed for them. To make this fully under- 

 etood, it will be necessary to explain that all the finer bearings of an instrument 

 made of aluminum, such as centers, object slides, leveling and micrometer screws, etc., 

 will have to be bushed with a harder and non-friction metal, to guard against friction 

 and wear and to obtain the close fitting of such parts, and permanency of adjust- 

 ments so necessary in instruments of precision. Now, to make the principal 

 bearings of an instrument of different metals will have the tendency to weaken the 

 parts so treated, to make them less secure, and to render the adjustments more 

 liable to derangement on account of unequal contraction and expansion between 

 the two metals. It simply means, then, that the present high state of perfection 

 in geodetic instruments, which retain their adjustment in the varying temperatures 

 and climes of our zone, shall be abandoned, and we go back many years to when 

 the indiscriminate use of widely different metals often made an instrument entirely 

 unreliable, except when used in the temperature in which it was adjusted. 



Modern instrument-making has, however, already achieved great results in re- 

 ducing the weight of field instruments. By improved designs and by the use of 

 harder metals in place of the soft brass, remarkable changes have been brought 

 about in the weight of instruments. They are no longer the heavy and formless 

 structures of soft or hammered brass as of yore, but are of the type and character 

 of a long-span steel bridge, as compared with an old-fashioned wooden structure. 

 Every important member of an instrument is now calculated with regard to its 

 strength, and the materials are particularly chosen for the part they are to perform. 

 Commercial Aluminum, unless obtained from reliable sources, of ten contains a small amount of iron. 



