TRANSACTIONS OF THE SECTIONS. SI 



with the first, had been found sufficient to make the arrangement capable of re- 

 sisting flexure and vibration in a surprising degree, and, as the angle may be reduced 

 so far that both bars can be attached to a slide carrying the eye- pieces, it is also 

 more convenient than the steadying wire which has been sometimes employed. 



The facility and accuracy offered by the use of the new collimator are such, that it 

 was suggested that in some instances it might be desirable to make arrangements for 

 adjusting the telescope after every considerable change of altitude. If the collimator 

 were to be thus frequently employed, a beautiful contrivance made use of by Lord 

 Rosse might with much advantage be adapted to it, by mounting it and one or two 

 of the eye-pieces in a slide so that any one of them could in a moment be brought 

 opposite to the cone of rays. A slide moving on a centre was recommended. Since 

 the tilt of the large mirror will thus become of less importance, we may henceforth 

 admit for its support arrangements which introduce more tilt than those at present 

 in use, and thus the solution of what now remains the most difficult problem of 

 large reflecting telescopes may possibly be facilitated. 



On Phcenomena recently discovered in the Moon. By J. Symons, M.A. 



On the reasons for describing the Moon's Motion as a Motion about her axis. 

 By the Rev. W. Whewell, M.A., D.D., F.R.S. 



Meteorology, 



On the Causes of Great Inundations. 

 By Thomas Dobson, B.A., of St. John's College, Cambridge. 



The principal special causes which tend to produce great inundations in a country 

 are, the inclination and the lithological character of the surface of the basins drained 

 by the rivers of the country. 



Where the subsoil of a river-basin is composed chiefly of porous and therefore 

 permeable materials, as oohte, loose gravel, &c., the rain will be absorbed almost as 

 fast as it falls, and will reach the river gradually, after returning to the surface 

 through springs. 



But where the subsoil is generally compact and impermeable, as clay, granite, &c., 

 the rain will flow over the surface with more or less velocity, according to the 

 greater or less inclination of the surface to the horizon. 



So far, therefore, as the geological character of a river-basin is concerned, the 

 tendency to inundation due to that basin will be measured by the difference between 

 the areas of the permeable and of the impermeable superficial strata, and by the 

 inclination of the sides of the basin to the horizon. 



The general cause of great inundations in the countries forming the western sea- 

 board of Europe, is the easterly progressive motion of the cyclones, or revolving 

 storms, of the North Atlantic Ocean. 



Starting from the Caribbean Sea and the Gulf of Mexico, and sweeping during a 

 considerable portion of their early course along the warm surface of the Gulf- stream, 

 they collect the vapours so copiously generated in southern latitudes and finally pre- 

 cipitate them on the high lands and mountain chains of Europe. In general, several 

 cyclones follow each other in rapid succession, so that the continued rains at length 

 saturate the earth, and floods and inundations ensue. 



I shall illustrate these remarks on the general cause of European inundations by a 

 brief meteorological history of the great inundations in France in October 1844, 

 October 1846, and in May and June of the present year. 



1844. — The tracks of the cyclones of October 1844 have been determined by 

 Mr. Redfield. The first passed over the West Indian Islands on the 1st and 2nd of 



