TRANSACTIONS OP THE SECTIONS. 29 



built with her head south-east or south-west, little if any effect would be produced by 

 heeling. When examining the magnetic condition of the ' Slieve Donard,' they were 

 surprised to find that the vertical was very nearly that which would give no effect 

 from heeling. Their able stipendiary Secretary (to whom is due the credit of drawing 

 up the two Reports already published) immediately suggested that her head could not 

 have been east when building, which we had taken for granted ; and on inquiry we 

 found that, on account of her great length, she had been built diagonally, with her head 

 south-east nearly. Although he believed that for practical purposes sufficient inform- 

 ation had been obtained, yet there were anomalies in their observations that rendered 

 the theories deduced unsatisfactory. This he believed arose from the rapidity with 

 which they were obliged to carry on their experiments, on account of the passing in 

 and out of ships through the docks, from which cause the inductive influence of the 

 earth had not sufficient time to complete its effect. It had been proposed to request 

 the aid of the Admiralty in allowing the Committee to experiment on one of Her 

 Majesty's iron ships, in some convenient place, for an unlimited time. 



On the Iris seen on the surface of Water. By J. J. Walker, A.M. 



This iris, in shape a more or less obtuse hyperbola, may be seen occasionally, in. 

 addition to the common rainbow, when a sheet of calm water lies between the spec- 

 tator and the rain-cloud. It is formed by pencils of variously-tinted rays, which, 

 after emerging from rain-drops, undergo reflexion at the surface of the water; and 

 was first described and mathematically discussed by the author in the Philoso- 

 phical Magazine for June 1853. 



The object of the present communication was to describe the phenomenon by the 

 aid of an illustrative sketch ; to point out the relation in which it stood to the se- 

 condary rainbow observed by Halley, in which the rays had undergone reflexion at the 

 surface of water before entering rain-drops ; and to suggest the correct mode of de- 

 lineating it in works of art. 



Astronomy. 



On the Present State and History of the Question respecting the Acceleration 

 of the Moons Motion. By G. B. Airy, M.A., D.C.L., F.R.S., Astro- 

 nomer Royal. 



It had been known, from the time of Newton, that the motions of the moon are 

 disturbed by the attraction of the sun, and that a great part of the effect is of the 

 following kind, viz. that when the moon is between the sun and the earth, the sun 

 attracts the moon away from the earth ; and when the earth is between the sun and 

 the moon, the sun attracts the earth away from the moon ; and thus, in both cases, 

 it tends to separate the earth and the moon, or diminishes the attraction of the 

 moon to the earth. There are sometimes effects of an opposite character ; but, on 

 the whole, the first described is predominant. If this diminution were always the 

 same in amount, the periodic time of the moon passing round the earth would be 

 the same. But it was found in the last century, by Halley and Dunthorne, that 

 the periodic time is not always the same. In order to reconcile the eclipses of the 

 moon recorded by Ptolemy with modern observations of the moon, it was necessary 

 to suppose that in every successive century the moon moves a little quicker than in 

 the preceding century, in a degree which is nearly represented by supposing that at 

 each successive lunation the moon approaches nearer to the earth by one inch. The 

 principal cause of this was discovered by Laplace. First, it had been shown by him 

 and by others, that the attractions of the other planets on the sun and on the earth 

 do not alter the longer axis of the orbit which the earth describes round the sun, and 

 do not alter the length of the year ; but they diminish slowly but continually through 

 many thousands of years the degree of ellipticity of the earth's orbit. Now, when 

 the earth is nearest to the sun, the decrement of attraction of the moon to the earth 

 (mentioned above) is greatest; and when the earth is furthest from the sun, that 

 decrement i3 least. It had been supposed that the fluctuations of magnitude exactly 



