292 PHILOSOPHICAL TRANSACTIONS, [aNNO 1787. 



periment now related, but by various other considerations, which I shall not now 

 insist on, as they are not sufficiently finished to be laid before this Society. I 

 shall only add that, among other things which may be illustrated by it, one is, 

 that all the planets may possibly be of the same heat; since, if the matter of 

 which Mercury consists was averse to the generation of heat in proportion to the 

 greater number of the rays of the sun it receives more than the Georgium Sidus, 

 they would be both of the same heat, notwithstanding their different distances 

 from the sun. I have already said, that I was led to an inquiry into the subject 

 by the effect it has on chemical operations. 



XXXI I I. On an Observation of the Right Ascension and Declination of Mer^ 

 cury out of the Meridian, near his Greatest Elongation, Sept. 1786, made by 

 Mr. John Smeaton, F, R. S., with an Equatorial Micrometer, of his own In- 

 vention and Workmanship ; and an Investigatioti of a Method of Allowing for 

 Refraction in such kind of Observations, p. 318. 



After noticing several contrivances in telescopes, &c. as preparatory to his ob- 

 servations, Mr. S. says, the morning of Sept. 23, about a quarter past 5 o'clock, 

 the air being clear and perfectly serene, it being then about an hour after Mer- 

 cury's rising, and near -|- of an hour before the rising of the sun, I very readily 

 found Mercury with the telescope, and when found could easily see him with an 

 opera-glass; and Mercury being then in a state of very little alteration of decli- 

 nation, I adjusted one of the declination wires to his apparent run, by making 

 him traverse the whole field. The observations were then taken as in the first 

 table; and in the evening I was lucky enough to get those of a Ceti and o 

 Tauri, intending to repeat the whole the next morning and evening. The next 

 morning proved cloudy, and so continued, that I saw the planet no more; but 

 in the evening of the 26th, I found the stars come again so near the same decli- 

 nation, that I was encouraged to continue the observation to see what change 

 would happen. It then came on bad rainy weather till the 30th, when I again 

 repeated the observation, and found the stars to come so near in declination that 

 I was fully satisfied of the stability of the instrument, so far at least as could 

 regard 24 hours: but as I was then appointed to go a journey, and could have 

 no other use for it, I locked the door of the observatory, leaving the instrument 

 in its position, that I might see what change would happen by the time of my 

 return; and was quite astonished to find, on the 13th of October, that it had 

 remained in a manner unmoved; for it had suffered no more apparent alteration 

 than what might occur by the errors of observing, and alterations of the clocks 

 and transit. 



Mr. S. then states his observations in several tables, not now necessary to be 

 reprinted; from all which he made the following deductions: 



