What then followed George is unable to describe in detail (что последовало затем, Джордж не может описать подробно). He has a confused recollection of having, immediately on starting (он смутно помнит, что сразу же после старта; confused — смущенный; сбивчивый, спутанный), received a violent blow in the small of the back from the butt-end of number five's scull (получил сильный удар в поясницу рукоятью весла номера пятого; butt-end — толстый конец; хвостовая часть), at the same time that his own seat seemed to disappear from under him by magic (и в то же время /почувствовал/, что его сиденье исчезает из-под него /словно/ по волшебству), and leave him sitting on the boards (и оставляет его сидеть на досках = на дне). He also noticed, as a curious circumstance (он также отметил как любопытную деталь; circumstance — обстоятельство; случай, деталь, подробность), that number two was at the same instant lying on his back at the bottom of the boat (что номер два в тот же миг лежал на спине на дне лодки), with his legs in the air, apparently in a fit (задрав ноги вверх, очевидно, в припадке).

received [r'si:vd] instant ['nstnt]

A particularly nervous boy was appointed cox, and the steering principle explained to him by Joskins. Joskins himself took stroke. He told the others that it was simple enough; all they had to do was to follow him.

They said they were ready, and the boy on the landing stage took a boat-hook and shoved them off.

What then followed George is unable to describe in detail. He has a confused recollection of having, immediately on starting, received a violent blow in the small of the back from the butt-end of number five's scull, at the same time that his own seat seemed to disappear from under him by magic, and leave him sitting on the boards. He also noticed, as a curious circumstance, that number two was at the same instant lying on his back at the bottom of the boat, with his legs in the air, apparently in a fit.

They passed under Kew Bridge, broadside, at the rate of eight miles an hour (они прошли под мостом Кью, бортом /вперед/, со скоростью восемь миль в час). Joskins being the only one who was rowing (Джоскинс был единственным, кто греб). George, on recovering his seat, tried to help him (Джордж, вернувшись на свое место, попытался помочь ему), but, on dipping his oar into the water, it immediately, to his intense surprise (но, когда он опустил весло в воду, оно немедленно, к его великому удивлению; intense — крепкий, сильный, глубокий, значительный /о чувствах, ощущениях и т.д./), disappeared under the boat, and nearly took him with it (исчезло под лодкой и чуть нет увлекло его за собой).

And then "cox" threw both rudder lines over-board, and burst into tears (потом «рулевой» бросил оба рулевых шнурка за борт и расплакался; to throw; rudder — руль).