HAEMONIC ANALYSIS AND PEEDICTION OF TIDES. 131 



machine is operated the height pointer will indicate the predicted 

 height of the tide corresponding to the time shown on the time dials. 



In order to increase the working scale of the machine when predict- 

 ing tides with smaller ranges, two additional circular height scales 

 are provided, one with the circle divided into 20 units and the other 

 into 10 units, with the units subdivided into tenths. These scales 

 may be easily removed or replaced on the machine, the scale in use 

 being secured in place by a small button at the top {9, fig. 12). The 

 20-unit scale may be conveniently used when the extreme range of 

 the predicted tide at any place is between 10 and 20 feet, and the 

 10-unit scale when the extreme range is less than 10 feet. If the 20- 

 unit scale is to be used, the value of each coefficient of both the cosine 

 and the sine terms must be doubled before setting the component 

 cranks, and if the 10-unit scale is used these original coefficients must 

 first be multiplied by 4 before setting the values in the machine. If 

 the extreme tide is less than 4 feet, the 40-unit dial may be readily 

 used as a 4-unit scale by considering the original unit graduations as 

 tenths of units in the larger scale. In this case the coefficients of the 

 cosine and sine terms of the formula must be multiplied by 10 before 

 entering in the machine. The factor used for multiplying the coeffi- 

 cients to adapt them to the different height scales is called the work- 

 ing scale of the machine. Working scales of 1, 2, 4, and 10 are now 

 in general use to take account of the different ranges of tide at the 

 places for which predictions are made. 



Predicted times of the tide. — Simultaneously with the summation 

 of the cosine terms of formula (472) on the height side of the machine 

 the summation of the sine terms of formula (474) , which was derived 

 from formula (473), is being effected on the time side. Being con- 

 cerned only with the time at which the sum of the sine terms is zero, 

 no provision is made for registering the sum except at this time, 

 which is indicated on the machine by the conjunction of the index 

 point on the time chain and the fixed platinum index in the dial case. 

 Near the time of a high water the index on the chain moves from right 

 to left and near the time of a low water from left to right. The con- 

 junction of the movable and fixed index is visible to the operator of 

 the machine and he may note the corresponding dial readings for the 

 time and height of the high or low water. 



Automatic stopping device. — This device provides for automatically 

 stopping the machine at each high and low water. Secured to the 

 hand-crank shaft is a ratchet wheel and just above the ratchet wheel 

 is a steel pawl {25, fig. 15) operated by an electromagnet (26) mounted 

 under the desk top. The electric circuit for the electromagnet is 

 closed by a contact spring that rests upon a hard-rubber cylinder 

 (31, fig. 16) on the rear end of the shaft on which the time summation 

 wheel is mounted. A small platinum plug in this rubber cylinder 

 comes in contact with the spring, which is fitted with a fine motion 

 adjustment, when the time summation chain registers zero. This 

 closes the circuit and draws the pawl against the ratchet wheels 

 thereby automatically stopping the machine. The lateral screw 

 motion of the shaft on which the rubber cylinder is mounted prevents 

 the platinum plug from coming in contact with the spring on any 

 revolution other than the one which brings the time chain to its zero 

 position. The circuit is led through an insulated ring on the hub 

 of the hand crank where a contact is kept closed by a spring. After 



