OBSERVATIONS OF THE MODEL. 



57 



preserved, I have since reduced the measures of cusps, by means of the 

 data given above, to see how they agreed with the true contact. The time 

 of internal contact was deduced from a cusp measure in the following 

 manner : 



The measurements of the model in inches were converted into their 

 equivalents in revolutions of the micrometer, by taking the diameter of the 

 planet as on page 54. 



The small quantity by which the limb of Venus was outside the limb of 

 the Sun, measured on the line of centers, was found from the equation given 

 on page 47, and a table was then formed giving the distance of cusps 

 corresponding to every ten seconds of time from true contact. The interval 

 of time between the measure and true contact (called for convenience the 

 reduction to contact), which rarely exceeded 250 seconds, was taken from the 

 table for every cusp measure separately. 



In order to test the accuracy of the data employed, more especially the 

 assumed velocity of horizontal motion of the planet, the driving apparatus 

 being of somewhat rude construction, I have separated the results of the 

 larger and smaller cusp measures. The following table exhibits these results 

 in sufficient detail. The telescopic contact was generally noted by one or 

 two observers in addition to the observer who took the measures. 



The table proves conclusively that the time of contact inferred from cusp 

 measures is 3 s or 4 s too late for my own observations, and 4 s or 5 s too late for 

 Mr. NICHOL'S, the probable cause being that the extreme points of the cusps 

 are not seen, although they appear so sharp. 



COMPARISON of the TIME of INTERNAL CONTACT at INGRESS, derived from CUSP 

 MEASURES of the MODEL with the OBSERVED TELESCOPIC CONTACTS. 



(1.) Observer, TUPMAN. 



