644 PHILOSOPHICAL TRANSACTIONS. [anNO l/Oi. 



gree of exactness sufficient to make it of great and valuable utility to the exten- 

 sive navigation and commerce of our native country. 



XCIF. Reasons for a Lunar Atmosphere, Bi/ Mr. Samuel Dunn. p. 578. 



It has been a question which has been long debated among astronomers, whe- 

 ther the moon has an atmosphere or not ? and the question it seems is not yet un- 

 decided. The best astronomers told him they never could discover any atmosphere 

 about the moon. But this being unsatisfactory, Mr. D. began to consider by 

 what methods, not already used, this problem was likely to be solved; and, 

 among several others, thought of one, which he thought had not been used be- 

 fore, viz. by a nice examination of the two ends of Saturn's ring, at the time 

 when the planet is on the dark edge of the moon. For the ring of Saturn being 

 of a considerable length, and gradually emerging or immerging almost at right 

 angles, either from or to the dark disk of the moon, the two extremities of this 

 ring, and the body of Saturn, being duly obsen^ed, if both the preceding and 

 subsequent extremities of the ring, and the body of Saturn also, should happen 

 to appear not perfectly defined, exceedingly near to the moon's dark limb, but 

 perfectly defined a little farther from it ; by such an appearance, he concluded it 

 might be strongly presumed, that there is a lunar atmosphere ; and for want of 

 such appearance, that there is none. 



Such an observation he made l6th instant, (June 1762) or rather 17th, past 

 2 in the morning ; the particulars of which follow. 



The 1 6th at noon he set a pendulum clock, by which this observation was 

 made, to solar time, by the sun's transit over the meridian. He waited to 

 make this observation, but could not see the moon till 14** 22"^, when she 

 emerged from dark still clouds into a most clear and serene sky, nothing could 

 be finer for observation; and thus she continued during the observation, and 

 long after it. The telescope being rightly adjusted, at 14** 21"* 3% he saw a 

 faint point of light, where the emersion afterwards appeared; but this faint point 

 of light appearing and disappearing by alternate fits, he could not know if it was 

 part of Saturn or of one of his satellites, till it was 14^ 21"* 13% when this 

 point of light was grown a little brighter and larger, and therefore he judged it 

 was the tip of the ring just emerging. Yet it appeared so dull and hazy, that he 

 had suspected his telescope, if he had not known it to have been rightly adjusted. 



At J 4^ 22"* 4% the preceding part of the ring was emerged, and it appeared 

 more bright; and now the body seemed emerging or emerged, but so very hazy 

 and ill defined, both the body and the ring confused together, closely on the 

 moon's dark limb, that he should not have taken it for Saturn, but for a comet 

 emerging from behind the moon, had he not known otherwise from the tables, 

 or seen Saturn the preceding mornings. 



