During rain the potential is usually but not always negative, and frequent alternations of sign are not uncommon.
It is subject, however, to extreme and rapid variations in temperature, to alternations of dry and humid winds (the latter, called catias, being irritating and oppressive), to chilling night mists brought up from the coast by the westerly winds, and to other influences productive of malaria, catarrh, fevers, bilious disorders and rheumatism.
There followed a long conflict, with alternations of success and defeat, which was not terminated till the death of the prince of Viana, perhaps by poison given him by his stepmother, in 1461.
This system of course requires that the exchange equipment shall include machines _ capable of delivering a positive pulsating current and a negative pulsating current, besides the usual alternations required for the ringing of ordinary subscribers.
The Jews of Hungary shared with their brethren in Austria the same alternations of expulsion and recall.