454 Intelligence aiid Miscellaneous Articles, 



From the result of these calculations it appeared that the place of 

 greatest intensity was not at the caustic but on its convex side, or 

 (for the rainbow) within the primary bow : that the intensity was 

 no where infinite ; that it diminished most rapidly on the concave 

 side of the caustic ; that on the convex side, after having increased 

 to a maximum, it diminished to 0, and then increased to a second 

 maximum, whose value M'as about |^ths of the first. The calculations 

 did not extend to the next evanescence of light. The following rule 

 was given for ascertaining the place of the geometrical rainbow (on 

 the theory of emission) : measure the distance between the bright 

 bow and the first spurious bow ; the geometrical bow is exterior to 

 the bright bow by -ir^ths of this quantity. 



LXXI. Intelligence a7id Miscellaneous Articles, 



GRESHAM COLLEGE. 



[As some of our correspondents have formerly endeavoured to call attention to 

 the subject of Gresham College, we have given the following brief notes of Pro- 

 fessor Pullen's admirable Lectures, and have much satisfaction in pointing out to 

 those who are desirous of attending to astronomical and physical studies a means 

 of improvement which is freely open to the pubUc. 



It is greatly to be wished that the Gresham trustees would second and encourage 

 the efforts of the Professors, by affording the lectures the advantage of the requisite 

 illustrations. The usefulness of Mr. Pullen's would have been very much increased 

 had he been suppUed with diagrams large enough to be seen by the audience.] 



ON Friday, April 20th, Mr. PuUen the Professor of Astronomy, 

 commenced a course of three lectures on the tides. In the first 

 lecture he explained the general phsenomena of tides according to 

 Bernoulli's, or the Equilibrium Theory. The earth being supposed 

 a sphere covered with water to a certain depth, the attraction of the 

 moon would have the effect of throwing that water into the form of 

 a prolate spheroid, of which the upper pole is raised by the excess 

 of the moon's attraction on the waters immediately subjected to her 

 influence over that on the general mass of the earth ; the lower re- 

 sults from a similar relative effect, the greater attraction of the moon 

 on the mass of the earth subtracting it from the water. This spheroid 

 will follow the moon at a certain interval ; and a spectator on the 

 earth's surface in the course of 24^48"^ (alunar day,) will be sensible of 

 two tides, one by a passage through the upper, another by a passage 

 through the lower pole. An effect similar to that of the moon is 

 produced by the sun, but in a less degree. There will, therefore, 

 be a tidal spheroid at a certain distance from the moon, and another 

 tidal spheroid of greater dimensions following the moon. These 

 eflfects in their combinations give rise to the semimenstrual inequal- 

 ity as well in the height of the tide as in the lunitidal interval. At 

 the moon's conjunction and opposition the two effects are combined, 

 and the result is spring tide ; when the moon is in quadratures, the 

 elevation due to one spheroid is partly counteracted by the depres- 

 sion due to the other, and the result is the phaenomena of neap tides. 

 The lecturer proceeded to show the effect which the monthly varia- 



