T. Hill on the Occultator. 301 
hourly motion of 5°6234 inches,—and calculate the proper 
values for semi-diameter and parallax, and direction of the 
meridian. 
(d) Other modes have been tried, but are not worth describing. 
It wi observed that the mode ais naturally suggested 
by the train of thought which led to the invention, 6 by the 
calculation and publication of Bessel’s coérdinates. The mode 
¢, requiring a modification of Bessel’s codrdinates, was invented 
by my son, H. B. Hill, who has calculated the occultations for 
the American Ephemeris. Its advantage is, that it saves the 
labor of dividing the moon’s hourly motion into minutes, which 
under the other forms is the most tedious part of the process. 
With the practice which he has had, he is now able to compute 
y this instrument the times of immersion and emersion, and 
the angle of emersion for sixteen different places of observation, 
in less than one hour, The times are supposed to be accurate 
— —— minute, unless the occultation is little more than 
appulse. 
The instrument was made by J. H. Temple, of Boston, who 
hour angle and latitude, I might add a fourth, parallax) on 
ova through a common point, the center of the model 
The above description of the process of invention sufficiently 
demonstrates the principle of the instrument, but it may be 
such that pC=Pz. Make the 
Pe Cps=zP§ and ps=pC. The two triangles, zp 8, 
_ #8, are evidently symmetrical, so that sz=Sz and the 
 angleszp=z8P. ‘That is to say, the altitudes of s and 8 are 
~ parallax of 8. But it is evident from the construction 
€ instrument that the steel rod points at s. 
