322 



Lieut.-Col. A. Strange gii a new Theodolite [May 2, 



three inverted radial grooves formed in the lower surface of a massive cast- 

 iron circle, which is thus supported by the three elevating screws without 

 being attached to them, and which is therefore able to accommodate itself 

 to expansional changes without restraint. The upper surface of the cast- 

 iron circle is turned flat and true, and receives the three feet of the instru- 

 ment. It has three lateral screws for centering the instrument over the 

 station-mark, as in existing large theodolites ; it has also three rollers, 

 capable of being pressed upwards by strong spiral springs thrown in and 

 out of action by a cammed lever, this arrangement, already in use else- 

 where, being intended to facilitate the setting of the horizontal zero. 



The Feet-screws. — These are, as usual, three in number ; no part of 

 such instruments has been so entirely neglected as this. Whilst the 

 Circles, Micrometers, and Levels indicated fractions of a second of arc, the 

 foot-screw remained so coarse and primitive in form that the due levelling 

 of the instrument was a matter of tedious difficulty, accomplished after all 

 by a sort of adroitness helped by accident. 



The first desideratum was that all clamping of the foot-screw after ad- 

 justment should be rendered unnecessary, since no clamping arrangement 

 hitherto tried could be used without disturbing the level after adjustment. 

 What was wanted, therefore, was that no azimuthal deviation of the mass 

 of the instrument should ensue from an undamped foot-screw, however 

 loose. 



The idea, as it presented itself to my mind, was that the effect which 

 would be produced by vertical expansion of the end of the tribrach should 

 be imitated if possible. After much thought I realized this idea by a very 

 simple contrivance, 

 the principle of 

 which the annexed 

 diagram will make 

 plain. The tribrach 

 arm is slit horizon- 

 tally near its lower 

 surface. This leaves 

 a kind of long flat 

 tongue of metal, which, being of considerable width, has great horizontal 

 rigidity, but, being comparatively thin, is easily bent vertically. The outer 

 end of the tongue rests on the stand ; the foot- screw passes through the 

 solid upper part of the tribrach, but not through the lower tongue. 



It is evident that when the screw is turned inwards with the screwing- 

 motion, the solid end of the tribrach will be raised, the slit being widened 

 by the vertical yielding of the tongue, and vice versa. But since the end of the 

 screw does not rest on the stand, but on an intermediate plate or tongue, which 

 is act-ally a portion of the tribrach itself, it is clear that if a lateral force 

 be applied to the tribrach, no motion will be caused thereby, however loose 

 the screw may be, so long as that force is less than the lateral rigidity of 



