459 



" Researches towards establishing a Theory of the Dispersion of 

 Light." By the Rev. Baden Powell, M.A., F.R.S., Savilian Pro- 

 fessor of Geometry in the University of Oxford. 



The author here prosecutes the inquiry on the dispersion of light 

 which was the subject of his former papers published in the Philo- 

 sophical Transactions for 1835 and 1836, extending it to media of 

 higher dispersive powers, which afford a severer test of the accuracy 

 of M. Cauchy's theory. He explains his methods of calcula- 

 tion and the formulae on which his computations are founded, and 

 which are different from those employed in his former investigations : 

 and then states the results in a tabular form. On the whole he con- 

 cludes that the formula, as already deduced from the undulatory 

 theory, applies sufficiently well to the case of media whose disper- 

 sion is as high as that of oil of anise-seed: or below it, such as 

 nitric, muriatic, and sulphuric acids, and the essential oils of angelica, 

 cinnamon, and sassafras, balsam of Peru, and kreosote. It also re- 

 presents, with a certain general approximation to the truth, the in- 

 dices of some more highly dispersive bodies. The author therefore 

 considers it as extremely probable that the essential principle of 

 the theory has some real foundation in nature. From the regularity 

 which he finds in the deviation of observation from theory, he thinks 

 it likely that the formula only requires to receive some further de- 

 velopement, or extension, in order to make it apply accurately to 

 the higher cases, while it shall still include the simpler form which 

 so well accords with the lower. 



" A few remarks on the Helm Wind." By the Rev. William Wal- 

 ton, of xlllenheads, near Hexham. Communicated by P. M. Rog;et, 

 M.D., Sec. R.S. 



On the western declivity of a range of mountains, extending from 

 Brampton, in Cumberland, to Brough, in Westmoreland, a distance 

 of 40 miles, a remarkably violent wind occasionally prevails, blow- 

 ing with tremendous violence down the western slope of the moun- 

 tain, extending two or three miles over the plain at the base, often 

 overturning horses with carriages, and producing much damage, 

 especially during the period when ripe corn is standing. It is ac- 

 companied by a loud noise, like the roaring of distant thunder: and 

 is carefully avoided by travellers in that district, as being fraught 

 with considerable danger. It is termed the helm wind ; and its 

 presence is indicated by a belt of clouds, denominated the helm bar, 

 which rests in front of the mountain, three or four miles west of its 

 summit, and apparently at an equal elevation, remaining immove- 

 able during twenty-four or even thirty-six hours, and collecting or 

 attracting to itself all the light clouds which approach it. As long as 

 this bar continues unbroken, the wind blows with unceasing fury, not 

 in gusts, like other storms, but with continued pressure. This wind 

 extends only as far as the spot where the bar is vertical, or imme- 

 diately overhead; while at the distance of a mile farther west, as 

 well as to the east of the summit of the mountain, it is not unfre- 

 quently almost a perfect calm. The author details the particulars 



