412 Mr. H. Darwin on 
oil-bath ; any one revolution of the sleeve X being accom- 
panied or unaccompanied by a closing of the shunt-cireuit 
according as the temperature of the copper arms of the 
bridge B was too low or too high. 
In order to make the regulation less spasmodic than it 
would be under such conditions, two furtber adjustments are 
introduced. In the first place, the series-coils included in the 
main circuit are automatically * diminished in resistance as 
the demand for heating becomes heavier, and wee versé ; and 
in the second place, the time during which the cireuit is 
closed is made to bear a greater or a less proportion to the 
period of revolution of the sleeve X, as the case may be. 
If diminished resistance in the series-coils and increased 
duration of short-circuiting are required, it is evident that 
both of these results may be accomplished by displacing the 
drum D and slider W along the spindle towards the right 
in fig. 1]. This, of course, implies an equal displacement of 
the cam (, since C, D, and W are all connected axially. 
It is, in fact, by means of the cam C that the displacement 
is effected. The cam acts both radially and axially, its 
radial action causing the rise and fall of the comb K, 
and consequently of the hit-or-miss arm H; while by its 
axial action, the cam itself, together with the drum D 
and slider W, is displaced endwise along the spindle 8S. The 
general form of the cam © is best seen in fig. 2; the view 
being that which is obtained by looking along the shaft 8 
from right-hand side in fig. 1. In this figure the measure- 
ments indicated in millimetres are the distances of the 
corresponding points of the periphery from the principal plane 
of the cam, that distance being marked + which is towards 
the observer, and vice versé. To make the action of the cam 
more readily comprehensible, the distinctive features of the 
two sectors are here set out in parallel columns. 

Sector C,. Sector Co. 
Has a larger radius. Has a smaller radius. 
Is left-kandedly helical, and so | Is right-handedly helical, and so 
traverses the cam, drum and | traverses the cam, drum and 
slider towards the left in | slider towards the right in 
fig 1: fie. 1. 
Has a throw of 4 mm. | Has a throw of 8 mm. 
Operates at every revolution of | Operates only when the hit-or-miss 
the sleeve X. | arm has missed the boom G,. 
It should further be observed that when the sector C, 
comes under the comb K, whether the arm H is up to that 
moment resting at the lower level upon the stop-screw, or at 
* The device for automatically varying the external resistance of the 
heating circuit, namely, the slider W “and resistance-blocks L, was 
suggested by Mr. Lunt, “of the Royal Observatory, Cape of Good Hope. 
