200 TRANSACTIONS OF THE 



local attractions. But the most careful adjustment may be ren- 

 dered useless by a thoughtless change in the position of the attrac- 

 tive forces. Many accidents are attributed to these compass 

 deviations. Especially in iron steamers are tliey to be dreaded. 

 The exhibiters purpose to envelope the compass needle with a 

 guard against these local attractions, that shall not at all aifect 

 its sensibility to the natural attraction of the magnetic pole. 

 One of the compasses was thus tested. Deprived entirely of its 

 patented addition, it was easily and immediately made to diverge 

 from its normal magnetic position by the presence of a file or 

 piece of steel, following it through an entire revolution. Part of 

 the patented addition was then applied; at least one-half of the 

 needle's susceptibility to local attraction was destroyed. It 

 required care to make it follow the file, and its movements were 

 very sluggish. Wlien the e?ifi}-e attachment was made, the steel 

 &c. used before had wo j^erceptible influence, even when brought 

 within half an inch of the needle, and only a large mass of iron, 

 brought quickly near, had an influence, and that very feeble. 

 Testimonials from navigators who had used this compass being 

 shown to us, we consider it an invention of inestimable value; 

 cheap in consU'uction, and of easy application. 



[^ gold medal awarded. 



Surveying histruments. 

 H. W. Hunter, 169 William street, New- York. 



This cut represents one of his transits for rail- 

 road and other surveying. The great value of 

 these instruments and tlie accuracy of tlieir 

 measurements of angles, are due to two things : 

 To the telescope, by which great precision in 

 rj sighting to a point is obtained ; and to the Ver- 

 nier scales, which enables minute portions of 

 an arc to be read with ease and correctness. 

 The telescope assists the eye in directing the 

 line of sight, and the Vernier scales in reading 

 'off the results. There are also arrangements 

 for giving slow and steady motion to the moveable parts of the 

 instruments, which add to the value of the above. There is also 

 a contrivanoe for repeating the observation of angles, which far- 



