186 



IOWA ACADEMY OF SCIENCES. 



ink. An injector or, what is more to the point, a force- 

 pump forces ink through the tube and into the pen. It is 

 operated by hand. Alouf^ the side of the dividing-engine 

 you may see a scale. A pointer from H shows the number 

 of complete turns of the screw. 



Figure 28 shows the engine as seen from the operator's 

 end; the mounting of the telescope between F and I; the 

 attachment of the gas-pipe between A and F; and at D 

 the pointer to the side of the engine. There are two 

 other pointers to be seen here. They are flexible pointers 

 for the divided head which is of a peculiar construction. 



Instead of having one divided circle it has three. This 

 was invented after half of the first zone plate was com- 

 pleted and its value is quite apparent from the striking 

 difference in the results obtained. In order to set the 

 engine the figures were taken from a table of square roots, 

 but these could not be used directly since the factor used 

 in this case was thirty. It was simple enough to multiply 

 by ten but to multiply by three involved the manipulation 

 of some thirty thousand figures. It was an immense task 

 but was cheerfully begun. The use of the new scale at 

 the side of the engine, combined with the new form of the 

 divided head, now^ makes it possible to set the engine directly 

 from the tables without the use of another figure. It may 

 be well to state here that the method of this divided head 

 may be applied in a general manner. The head may be 

 divided into any number of circles, not necessarily three, 

 but the complementary scale, H, figure 24, should be 

 divided into the same number of parts for direct reading. 

 If you will look at this scale, H, you will see that each 

 divisions represents a complete turn of the screw and 

 every third division is numbered in succession. Suppose, 

 for instance, you were! to set the engine at 11. 85. You 

 would turn until the pointer on tiie scale, H, stood at 11. 00. 

 Now you would look at the divided head and use the outer 

 circle, but you would not find .85 on this. It reads from 

 to .33|. Turn the screw one complete revolution and 

 your pointer on H would be on the next division which is 

 not marked. Look at the divided head once more and use 



