198 A New Angle Measurer. 



At A, a needle point passes through the side of the tube to 

 furnish an index, as before stated. 



The other end of the tube carries a lens of 1-^ focus, for 

 viewing the needle point and divisions of the wheel. Sorae 

 time since I gave Mr. Elliott, optician of the Strand, permission 

 to make similar instruments if he wished to do so. He may 

 possibly have some in stock, but of this I am not informed. 



For measuring a short base line I use, with the angular 

 instrument, a light rod of deal, 5 feet in length and § inch 

 square. This is attached by a joint at the centre to a walking 

 stick.* A short base line is readily measured by this — merely 

 marking by a scratch on the ground each length of 5 feet as 

 you proceed. To register the number of lengths, I have a 

 small toothed wheel, 2^ inches in diameter, which is attached to 

 a foundation plate by a screw in its centre. A spring catch 

 enters the teeth. Thus, by moving a tooth for each ten or 

 other lengths of the measuring rod, an accurate account is 

 kept. Instead of the toothed wheel an account may be kept 

 by shifting a number of marbles from one pocket to the other. 



We hope the following hint may also be of service. Having 

 made one of the reflecting levels (a very useful pocket-com- 

 panion) described in Heather's Treatise on Mathematical Instru- 

 ments, p. 120 (Weale), I had difficulty in making the line 

 scratched on the mirror perfectly horizontal. This difficulty may 

 be obviated by adopting a circular instead of a square shape for 

 the centre of the instrument. In this a cell for the mirror 

 may be turned, and it can then be adjusted with the greatest 

 accuracy. 



[We think Mr. Heineken's ingenious instrument might be 

 improved by enabling one eye to view the object and read off 

 the altitude on the scale. If the wheel and its supports occu- 

 pied half the tube, and a knife edge or needle were stretched 

 right across it, the eye, looking through a hole drilled in the 

 centre of the lens at the eye-piece, would see the object, and 

 be able to bring up the instrument so that it coincided with 

 the knife edge, and the same eye looking through the lens 

 would read off the magnified image of the scale. In this 

 modification it would be better to have the tube square, as the 

 wheel might then come very close to one side. Mr. Heineken 

 finds that the error introduced by looking through the instru- 

 ment with one eye, and at the object with the other, is very 

 small, but we think, with most observers, it would be less 

 with the modification suggested.] 



* Mr. Heineken lias sent us a drawing of this contrivance. The walking stick 

 and the measuring rod, when not in use, fold together ; when in use they open 

 like an inverted capital T (±), and by holding the walking stick in the band, and 

 pressing the rod to the ground, the measure can be taken without stooping. 



