102 Description of a Mercurial Pendulum. 



fraction, it being so totally different when the occultation took 

 place at the moon's dark limb. If the moon has an atmosphere, 

 its existence is not, I think, likely to be discovered bv such ob- 

 servations; as tlie rays of light coming from a star to an ob- 

 server on the earth's surface, will not suffer any perceptible change 

 in its direction from passing through it, although its density should 

 be as great as at the earth's surface. The only difference in the 

 appearance will be in the quantity of light, which difference is 

 perceptible ; but whether it arises wholly from the star being near 

 to the moon, or partly from the light of the moon and partly 

 from a lunar atmosphere, remains yet to be decided ; and per- 

 haps one of the best means to decide this doubtful point, would 

 be to make accurate observations on the approach and recession 

 of stars towards and from the moon's dark and enlightened limb in 

 occultations: but for this pur})ose a much greater number should 

 be computed than is usually inserted in the Nautical Almanac. 



XIX. On the Compemation Mercurial Pendulum of Mr. Gavin 

 Lowe. Communicated by Thomas Firminger, LL.D. 



A HE late Mr. Gavin Lowe of Paradise-row, Islington, a gen- 

 tleman eminently skilled in the mathematics and in the know- 

 ledge of theoretical and practical astronomy, is known to have 

 made some years since a considerable improvement in the quick- 

 silver pendulum, by which it may easily be adjusted for the effect 

 of temperature, without removing it from the clock to which it 

 is attached, — an advantage which Graham's and the other forms 

 of compensation pendulums do not possess. Mr. Lowe, having 

 taken great pains to compute the dimensions of the several parts 

 of his pendulum from the best tables of expansion of steel, glass, 

 and quicksilver, the materials of which the pendulum is com- 

 posed, and also the quantity of quicksilver necessary to keep the 

 centre of oscillation stationary in all the variations of temperature, 

 applied it to his own transit clock ; and finding the pendulum to 

 perform to his expectation, he recommended it to a few time- 

 keeper makers of his acquaintance, who made several pendulums 

 from his directions. As the construction is simple, and the pen- 

 dulum easily made by any ingenious workman, and as trials of its 

 performance have shown that the clocks to which it has been ap- 

 plied, have L!,one as well with it as with anyotherkind of compensa- 

 tion pendulum of more complicated and expensive construction ; 

 and likewise as no improvement appears to have been made upon 

 it since the death of the inventor, it seems to be the only kind of 

 quicksilver pendulum now in general use. After having simpli- 

 fied his pendulum as much perhaps as it will admit, Mr. Lowe 



wrote 



