No. 129.] 205 



centre of the main needle. These small needles have brass 

 arms or pointers fastened to them, Avhich reach inward toward 

 the centre of tlie main needle, and move over semi-circular gra- 

 duated scales. These scales are graduated both ways from their 

 centres outward, the nonius being in the centres. 



The three needles are so charged and constituted to each other, 

 that when the main needle points in the magnetic meridian, the 

 pointers stand on the nonius of their respective scales. But 

 when the main needle is moved from the magnetic meridian by 

 any course, either to the east or the west, the pointers will as- 

 sume positions on the scales toward the letters at their extremi- 

 ties, either E. or W., these letters indicating the character of the 

 deflection, and the figures over which they stand being added 

 together and then halved, give the amount. 



This instrument shows promptly and actually the effect of all 

 local causes, whether magnetic or mechanical forces, in carrying 

 the main needle from the magnetic meridian. 



Mr. St. Johns' Velocimeter consists of a frame made to rise and 

 fall at pleasure, in a pipe inserted through the bottom of a ship, 

 in which the water runs equal to the draft of the vessel. This 

 movable frame covers a spiral wheel, which, dropping below the 

 bottom of the vessel, is made to revolve by the motion of the 

 vessel through the water. The revolutions of this wheel are 

 carried by shafts and wheels up to a train of groovipgs in the up- 

 per part of the vessel, covered by a dial over which the hands 

 revolve, one of which goes round once in one mile; the next 

 goes round once in one hundred miles; and the third goes round 

 once in one thousand miles. This instrument, under test at the 

 Washington navy j^ard, has been ceitiiied as an accurate mea- 

 surer of the velocity of the ship at any tim.e and the true distance 

 traversed. 



