338 Observations on the Flexure 



moon would have no centre to revolve about, and consequently 

 would fly off in a tangent. The tide on the opposite side to 

 the moon is occasioned by the centrifugal force which neces- 

 sarily arises from the revolution of the earth and moon round 

 their common centre of gravity, on the same principle that 

 the waters at the ecjuator are elevated from the centre by the 

 diurnal revolution of the earth on its axis. 



Captain Forman has made assertions in his postscript to 

 which, of course, I cannot assent. I shall, however, notice a 

 peculiar sort of convenience in which the Captain so freely 

 indulges ; namely, that of deciding without considering. He 

 says, " Mr. Russell, I suppose, is so nuicli of a philosopher as 

 to know that whenever it is low water in any place, the tides 

 are rising on one side of it and ebbing on the other ; and if 

 the 'superior gravity' of the water in this place cannot pre- 

 vent the waters from ebbing on one side, it is not very philo- 

 sophical to suppose that it can lift the waters on the other." 

 Captain Forman would have saved himself the trouble of this 

 last assertion, if he had previously considered that the earth 

 is continually turning on its axis ; and consequently is in re- 

 gular succession exposing every degree of the equator on one 

 side to the centrifugal force, and on the other side to the at- 

 tractive power of tlie moon ; and as the most dense waters 

 (the ebbs) necessarily form an isosceles triangle with the moon, 

 so are they ever ready to buoy up the flows. 



I now take leave of this controversy, and promise in some 

 future number of your Magazine to lay before your readers 

 some observations on the expansibility of water as connected 

 with the rising of the ocean. Henry Russelu 



LXIV. Observations on the Flexure of Astronomical Instru- 

 ments. By Mr. Thomas Tredgold, Civil Entrineer*. 



A HE further improvement of the accuracy of astronomical ob- 

 servations being of great importance to the advancement of 

 that science, a few remarks on the flexure of the parts of in- 

 struments may perhaps be regarded with interest by some of 

 your readers. 



A slight acquaintance with the properties of natural bodies 

 must have informed even a careless observer, that no kind of 

 matter is endowed with perfect inflexibility ; and on a closer 

 examination we find that every change in the position or 

 points of support of a body, is accompanied by a correspond- 

 mg change in its structure : the different parts of the body be- 

 * Communicated hy the Author. 



come 



