THE TIDES 309 



P, the solar diurnal tide. The last disk on the extreme 

 right is simply Professor James Thomson's disk-globe-and- 

 cylinder integrator, applied to measure the area of the 

 curve as it passes through the machine. 



The idle shafts for the M and the O tides are seen in 

 front respectively on the left and right of the centre. 

 The two other longer idle shafts for the K and the P tides 

 are behind, and therefore not seen. That for the P tide 

 serves also for the simple integrator on the extreme 

 right. 



The large hollow square brass bar, stretching from end 

 to end along the top of the instrument, and carrying the 

 eleven forks rigidly attached to it, projecting downwards, 

 is moved to and fro through the requisite range by a rack 

 and pinion, worked by a handle and crank in front above 

 the paper cylinder, a little to the right of its centre. Each 

 of these eleven forks moves one of the eleven globes of 

 the eleven disk-globe-and-cylinder integrators of which 

 the machine is composed. The other handle and crank 

 in front, lower down and a little to the left of the centre, 

 drives by a worm, at a conveniently slow speed, the solar 

 shaft, and through it, and the four idle shafts, the four 

 other tidal shafts. 



To work the machine the operator turns with his left 

 hand the driving crank, and with his right hand the trac- 

 ing crank, by which the fork-bar is moved. His left 

 hand he turns always in one direction, and at as nearly 

 constant a speed as is convenient to allow his right hand, 

 alternately in contrary directions, to trace exactly with 

 the steel pointer the tidal curve on the paper, which is car- 

 ried across the line of to-and-fro motion of the pointer by 

 the revolution of the paper drum, of which the speed is in 

 simple proportion to the speed of the operator's left hand. 



The eleven little counters of the cylinders in front of the 

 disks are to be set each at zero at the commencement of an 

 operation, and to be read off from time to time during the 

 operation, so as to give the value of the eleven integrals for 

 as many particular values of the time as it is desired to have 

 them. 



A first working model harmonic analyser, which served 



