VOL. LXX.] PHILOSOPHICAL TRANSACTIONS. 719 



sun from the moon without any material error, and which therefore we may find 

 at any given time from an ephemeris. 



Mr. H. then gives an account of his own observations relating to the moun- 

 tains in the moon. The telescope he used was a Newtonian reflector, of 6 feet 

 8 inches focal length, to which a micrometer was adapted consisting of two 

 parallel hairs, one of which was moveable by means of a fine screw. The value 

 of the parts shown by the index, was determined by a trigonometrical observa- 

 tion of a known object at a known distance, and was verified by several trials. 

 The power he always used, except when another is mentioned, was I'll times, 

 also determined by experiment, which he had often found to differ somewhat from 

 theory, on account of some small errors in the data, hardly to be avoided. The 

 moon having sufficient light, he used no more aperture of the object speculum 

 than 4 inches. 



Observations. — Nov. 30, IJjQ, 6 o'clock in the morning, a rock, situated 

 near what Hevelius calls Lacus niger major, was measured to project 41 '.56. 

 To reduce this quantity into miles, put r for the semidiameter of the moon in 

 seconds, as given by the Nautical Almanac at the time of observation, and a for 

 the observed quantity, also in seconds and centesimals; then it will be in general, 

 R : logo :: Q : on, in miles. Thus it is found that 41". 56 is 46.79 miles. This 

 distance of the sun and moon at the same time was, by the Nautical Almanac, 

 about 93° 57'J.. The sine of which to the radius 1 is .9985 &c. and — in this 



case, is lm = 46.85 miles. Then by Hevelius's method the perpendicular • 

 height of the rock is found to be about 1 mile. The same morning, a great 

 many rocks, situated about the middle of the disc, projected from 25".Q3 to 

 26'.56. This gives on about 29.3 miles, and these rocks are all less than half a 

 mile high. 



Jan. 13, 1780, 7 o'clock, Mr. H. examined the mountains in the moon; but 

 there was not one of them that was fairly placed on level ground, which is a 

 condition very necessary for an exact measurement of the projection. If there 

 should be a declivity on the moon before the mountains, or a tract of hills placed 

 so as to cast a shadow on that part before them which would otherwise be illumi- 

 nated, it is plain that the projection would appear too large; and, on the con- 

 trary, should there be a rising ground before them, it would appear too little. 

 As far as he was able to judge of the direction of the line of illumination, the 

 highest hill projected 26".31, or 30.36 miles: thence we find, as before, that 

 the perpendicular height (.42 mile) is less than half a mile. 



Jan. 14, 11 o'clock, betook the projection of the highest mountain, which 

 was situated at the western edge. It measured 24".68, or about 27 miles; and 

 the perpendicular height comes out less than half a mile. There was not one 

 mountain in the edge of the disc so high as this. Jan. ] 7, 7 o'clock, a very 



