During regular migration birds start in favourable weather but frequently meet with unfavourable weather before their arrival at the point aimed at; most of the bird "disasters" at the lighthouses and lightships, and more occasionally inland, can be explained in this way. In his digest of the observations at lighthouses and lightships Mr Eagle Clarke shows that spells of genial weather are favourable and that during these spells migration is even flowing and continuous (15). Slightly unsettled conditions have little effect, but an increase of the irregularities accelerates migration. Sooner or later cyclonic disturbances interrupt regular movements, and, if these are extraordinary, act as barriers, either holding the birds in one place or forcing a hurried departure or "rush." Favourable weather immediately following a check or "hold up" often causes a rush; a sudden fall in temperature may force large numbers of birds on in autumn or retard them in spring. Temperature, he declares, is the main controlling factor in all extraordinary movements, other meteorological conditions being suitable. In the autumn migration to Britain, the chief movements take place when a large and well-defined anticyclone has its centre somewhere over Scandinavia, with gentle gradients in a south-westerly direction over the North Sea. Coincident with this we usually find cyclonic conditions prevailing to the west of the British area, with low-pressure centres off the west or south-west of Ireland. The weather is clear and cold, with light variable airs over Scandinavia, but in Britain the sky is overcast, and the wind easterly and moderate to strong; not infrequently these conditions mean fog on our eastern coasts. If the birds leave Scandinavia under favourable conditions they may be met by the approaching cyclonic system, which usually, though by no means always, travels in a north-easterly direction across the Atlantic. Migration is thus checked, but a return of favourable anticyclonic conditions starts the birds again, often with a fresh impulse in the shape of falling temperature. When the anticyclonic area is exceptionally large, extending from the Scandinavian peninsula in a south-westerly direction and embracing the whole of the British Islands, simultaneous immigration and emigration may be witnessed. Cyclonic spells are not always unfavourable to migration. In spring, when they are of a mild type with soft rain and warm winds following after a cold anticyclonic period, a northward movement is frequent.