VOL. LXX.j PHILOSOPHICAL TRANSACTIONS. 717 



The journal of the barometer, which Dr. S. annexed, showed that its greatest 

 variation is about 30°, ranging between 6o°, the lowest state, and 92°, the 

 highest. 



XXIX. Astronomical Observations relating to the Mountains of the Moon. By 



Mr. Htrschel. p. 507. 



When the telescope was first invented this noble instrument was immediately 

 applied to astronomical observations with the most surprizing success. Several 

 eminent persons have given us an account of their discoveries ; and, notwith- 

 standing the imperfect state of telescopes in those times, we still owe a great deal 

 of our knowledge of the heavenly bodies to the observations that were made by 

 those first telescopic observers, who made amends for the deficiencies of their 

 instruments by their uncommon diligence and attention. It may perhaps be es- 

 teemed a mere matter of curiosity to search after the height of the lunar moun- 

 tains. Mr. H. allows that there are more necessary and more useful objects of 

 inquiry in the science of astronomy ; but when we consider that the knowledge 

 of the construction of the moon leads us insensibly to several consequences, 

 which might not appear at first ; such as the great probability, not to say almost 

 absolute certainty, of her being inhabited, we shall soon agree that these re- 

 searches are far from being trifling. 



Mr. H.'s reason for repeating observations that have been made by very good 

 astronomers, was not that he doubted either their veracity or diligence. The 

 names of Galileo, Hevelius, Kircher, and several more, will always deserve to 

 be mentioned with particular respect for the eminent services they have rendered 

 to astronomy ; but as we know that their instruments were far from being arrived 

 to that degree of perfection we have now obtained, he thought it by no means 

 improper or useless to repeat their observations on the lunar mountains, and to 

 extend them to other parts of the moon's visible hemisphere, and thus to esta- 

 blish this theory on the firmest evidence of a survey taken by a very excellent 

 instrument. 



The method used by Hevelius and others, to find the height of a mountain in 

 the moon, is this : let a ray of light slm, fig. 6, pi. 7, proceeding from the sun, 

 pass by the moon at l, and touch the top of a mountain at m : then the space be- 

 tween L and M will appear dark, and the top of the mountain will be seen to 

 stand at some distance from the illuminated part of the moon's disc. With a 

 good micrometer let the distance lm be taken by observation. Draw lc perpen- 

 dicular to LM ; draw also mc from the top of the lunar mountain to the centre 

 of the moon : then in the triangle mlc, right-angled at l, we have given the 

 side lc, which is the moon's radius, and the side lm taken by observation. 

 Therefore, by trigonometry, we can find the hypothenuse mc, from which 



