i8o POPULAR LECTURES AND ADDRESSES. 



worm carried on an intermediate shaft, with a 

 toothed wheel geared on another on the solar 

 shaft. In front of the centre is the paper drum, 

 which is on the solar shaft, and goes round in the 

 period corresponding to twelve mean solar hours. 

 On the extreme left, the first pair of disks, with 

 globes and cylinders, and crank shafts with cranks 

 at right angles between them, driving their two 

 cross-heads, corresponds to the K v or luni-solar 

 diurnal tide. The next pair of disk-globe-and- 

 cylinders corresponds to M, or the mean lunar 

 semi-diurnal tide, the chief of all the tides. The 

 next pair lie on the two sides of the main shaft 

 carrying the paper drum, and correspond to S, 

 the mean solar semi-diurnal tide. The first pair 

 on the right correspond to O, or the lunar diurnal 

 tide. The second pair on the right correspond to 

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

 extreme right is simply Professor James Thom- 

 son'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 



