Proceedings. 115 



Pro. Man. Lit. and Phil. Soc, Vol. II., 4th Series), the line 

 normal to the instantaneous surface is written in the form 



X-x_Y-y_Z-z 



c c c 



the equations should be 



X-* Y-y Z-2 



cosa cos/3 cosy 

 a, (5, y being the direction angles. 



Mr. J. J. ASHWORTH enquired whether any explanation 

 had been given of the cause of formation of the " Levanter 

 Clouds " at Gibraltar which he had lately seen. During the 

 continuance of the east wind blowing from the Mediter- 

 ranean Sea — a dense black cloud is formed to the west of the 

 rock of Gibraltar over the town and bay. This cloud can be seen 

 forming from below, as there appears to be for a distance of 

 some 40 or 50 yards from the top edge of the rock a 

 perfectly clear space through which the sky can be plainly 

 seen before the cloud begins to appear, and about half-way 

 across the bay the cloud seems to be absorbed again, having 

 a rather curious effect. The vapour of which the cloud consists 

 appears to be in continual motion from east to west. Early 

 in the morning there is very little of it to be seen, but it 

 gradually increases in density up to the middle of the day, 

 when it appears perfectly black, and hangs like a pall over 

 the town and bay, having a most depressing effect on the 

 inhabitants of the rock. Mr. Ashworth added that he 

 understood that it was found to be very unhealthy, as the 

 sick rate always increases during its presence. There is very 

 little, if any, deposit of rain, and the streets and town are 

 smothered with fine dust, which renders the presence of this 

 " Levanter cloud " very obnoxious. 



The President explained generally the formation of 

 clouds on the lee side of a hill, as caused by the existence 

 of a point of minimum pressure at some point below the 



