648 Prof. Pollock: Comparison of Periods of 
parallel wire system in tune with each other, and keeping 
this part of the apparatus fixed, to alter the length of the 
third cireuit by successive steps of 10 or 20 cms. From a 
plot of the observations of the disturbances in the third 
circuit, its length when it is in tune with the rectangle may 
‘be found. ‘This method is more satisfactory than the former 
one in that each successive step involves the alteration of 
only one of three circuits, instead of two as in that case. 
In repeating the experiments by this method, the plots of 
the observations showed more decided maxima. When the 
repetition was practically completed, owing to greater accard- 
ance among the observations due to the improvement in the 
character of the spark resulting from greater experience in 
the preparation of the spark knobs, two maxima close together 
were noticed in the plot of the observations with one of 
the straight wires. As the position of the maxima could 
be altered by changing the condenser-plate distance, the 
observation pointed to a want of success in tuning the con- 
denser and rectangular circuits, an operation of considerable 
difficulty, and threw doubt on the accuracy of all the previous 
work. 
It was decided therefore, after consideration, to do away 
with the hollow boxes attached to the rectangular circuit, 
which complicate the operation of tuning, and simply place 
the narrow end of a fixed rectangle near the spark-gap of the 
condenser circuit. The tuning of the condenser and rect- 
angular circuits now involved only a change of the condenser- 
plate distance. This operation was performed for three 
rectangles of different perimeters. It was found, however, 
that a want of tune between the condenser and rectangular 
circuits could be better detected by observations of the dis- 
turbance in the third circuit than of thatin the rectangle itself. 
The following procedure was finally adopted as giving the 
most definite results :—For a given distance between the 
condenser-plates, observations are taken of the disturbances 
in the third circuit while altering its length by successive 
steps. This series of observations is repeated for various 
distances between the condenser-plates. The plots of the 
observations show the relation between the disturbance in 
the third circuit and its length for various condenser-plate 
distances. That distance between the condenser-plates in 
connexion with which the plot shows the most definitely 
marked maximum, and in which the observations are perfectly 
symmetrical round this point, is taken as the distance which 
makes the condenser circuit in tune with the rectangle. 
This operation of tuning the circuits is a tedious one ; it 
has involved in some cases the comparison of fifteen complete 
