TIDE-PEEDICTING MACHINE NO. 2. O 



epochs of the components, modified to correspond to the beginning of 

 tlie period for which the predictions are to be made. 



The machine is operated by turning a large hand crank, wliich by a 

 system of gearmg, sets in motion all the component cranks to which 

 reference has been made. The pins attached to these smaller cranks 

 raise and lower a system of shding frames, the motion of which is 

 strictly harmonic. To each frame is attached a smaU pulley A 

 slender chain, one end of which is fixed, passes alternately downward 

 under one pulley and upward over the next, connecting all, and trans- 

 mitting the resultant motion of all the components to a dial in the 

 face of the machine, where the height of the tide for any given time 

 may be read directly. Other dials on the face of the machine indicate 

 the time to which the heights correspond. As the machine is being 

 operated, the time pointers pass rapidly around the dials indicating 

 successive mtervals of time, and simultaneously the height pointer 

 moves back and forth over a circular scale, indicating the corre- 

 sponding height of the tide. The height may be read on the dial 

 at any time desired. The predictions as directly given in the tide 

 tables are limited to the high and low watere, and the times and 

 heights of these are the only ones that are usually recorded l,v the 

 operator. •' 



The machine also constructs automatically a curve, which graphic- 

 ally represents the tide and which may be used to verify the hijrh 

 and low waters as tabulated by the operator, and also to obtain the 

 height ot the tide at any intermediate hour that may be desired 

 Tins curve is dra^vT^ on a roll of paper about 6 inches wide and about 

 3/0 feet long, which is sufficient for the record for one year at the 

 station. This paper moves through the machine at the rate of 1 

 foot for every day of record. A fixed pen draws a line representing 

 mean sea level near the middle of the paper, and at the beginning of 

 each hour niakes a little jog in the line. Another pen moves up tnd 

 down with the rising and falling of the tide, and this, together with 

 the ;™d motion of the paper, produces a tide curve from which 

 the height of the tide referred to mean sea level may be obtained for 

 any desired time. 



Another important feature of the machine is that which utilizes 

 the fact well known m mathematics that the &-st derivative of a 

 function becomes zero when the function itself is at a maximum or 



of the tides. The firet derivative of the harmonic terms representing 

 ilt.t^^f '\^ '™-''''' T* "^ ^^™'^°^° ^^^'^ ^'*h initial points 90° 

 tlTJl""' °"^™'^ ^'™'- ^''^''^^^ ^ ^^^ «^°^e shafts con- 

 sidHf It Z'^l''"''^' cranks ah-eady referred to, but on the opposite 



right angles to the other. On this side there is a similar set of pins, 

 4503°— 15 2 



