One of the most remarkable meetings of the Revival was held on July 12, 1859, on Frongoch Hill, three miles from Ysbytty Ystwyth. It was a thanksgiving service for the salvation of so many thousands. When the people gathered in the morning the heat was intense, the sky like a furnace above, and the earth a hotbed beneath. An old saint named Isaac Rees besought God to draw the blinds over His window, lest the blaze of the physical heat should counteract the power of the spiritual heat. "A cloud of thick mist," says the Diary, " came over the mountain; it shadowed the sun, and the earth cooled pleasantly, and heaven's influences descended so wonderfully that scores were rapturously ' rejoicing.' The cloud of mist remained till half-past three; extraordinary heat prevailed for the last half-hour of the service. There were 3000 present. It w^as a most striking scene when they all (at my request) fell on their knees for five minutes' silent prayer. Heaven manifested its pleasure in the service in a marvellous way. Those present w411 remember it while on earth." This thanksgiving prayer-meeting was repeated on the same spot annually till 1899.
From, 'The '59 Revival', by J J Morgan, page 73.
I am not sure exactly where the hill is.