Salter's advert in Thames Map by Henry Taunt, 1873

John Hawks Clasper (son of Harry Clasper)
advert in Thames Map by Henry Taunt, 1873

1870: BOATRACE - Cambridge by one and a half lengths. See Boatrace 1870s
1870: HEAD OF THE RIVER - University College

It was becoming quite clear that efficiency in rowing required the longer stroke. The old watermen's choppy quick stroke got boats off to fast starts - but time after time over a boat race course it was the boat with the slower steadier longer stroke that lasted the course.
But there is a limit to the longer stroke. You can lean right forward at the start (but everybody knows that is not a good way to lift a weight with your back bent), and you can lie right back at the finish, but unless you can somehow move your bottom that is the limit to the inboard travel of an oar.
If only you could use the extension and power of the legs!
And so in 1870 some Yale oarsmen appeared wearing greased leather pants. They locked their feet in place and slid back and forth on smooth wooden planks, incorporating leg power into their rowing stroke and increasing the arc of the oars. And it worked!
So they then developed a sort of tea tray which slid - which must have come as a fundamental relief!
J.C. Babcock described these seats in December, 1870 -

a wooden frame about ten inches square, covered with leather, and grooved at the edges to slide on two brass tracks fastened to the thwart, of sufficient length to allow a slide of from ten inches to a foot, though in rowing the proper length of slide is from four to six inches. The ways to be occasionally lubricated with lard, and I found no stops or fastenings necessary to keep the seat in position.
The slide properly used is a decided advantage and gain of speed, and the only objection to its use is its complication and almost impracticable requirement of skill and unison in a crew, rather than any positive defect in its mechanical theory.

Illustrated Catalogue and Oarsman's Manual For 1871 -

The sliding seat changed the sport from one of fairly simple upper body power with static leg pressure to one of very complex power application.

The physics of rowing a light weight boat with the much greater weight of the oarsmen travelling up and down the boat, causing the boat to accelerate and decelerate whilst also pitching stern down and then bow down, whilst levering the boat along against a liquid fulcrum is indeed fairly complex!
There are many crews who would (and a good few more who ought to) recognise Babcock's comment that "the objection to the moving slide is its complication and the almost impracticable requirement of skill and unison in a crew".

1871: BOATRACE - Cambridge by 1 length. See Boatrace 1870s
1871: HEAD OF THE RIVER - University College

1872: BOATRACE - Cambridge by 2 lengths. See Boatrace 1870s
1872: HEAD OF THE RIVER - Pembroke
1872: The Oxford Boatrace Crew, Practising on the Isis during the floods -

1872: Oxford boatrace crew in floods
1872: The Oxford Boatrace Crew, Practising on the Isis during the floods

1873: BOATRACE - Cambridge by 3 lengths. See Boatrace 1870s
This was the first boat race to use sliding seats. To those who have known nothing else it is strange that slides could be adopted without there being any abrupt change in style. The answer is that the length of slide was at first only 4 or 6 inches. And also there was some impact on style - but it facilitated the movement that had been going on anyway - the lengthening of the stroke. Cambridge took to it 'like a duck to water', Oxford found it more problematic. But then in the cyclic rise and fall of boat race fortunes Cambridge were at that time in the ascendant. The Oxford comment on this was:

Sliding seats were first used, but it was many years before it was discovered how to get the full benefit out of them. In the early days, as we find recommended in the president's book, the object of the coach was to get the men to forget they had a slide, and to swing naturally as if on a fixed seat.

1873: HEAD OF THE RIVER - Balliol

A fearful scrimmage occurred among the upper crafts. Balliol had bumped Trinity, and failed to get out of the way of Lincoln, who were driven against the wall by Queens. Wadham in hot pursuit, cut right into Queens, and were themselves upset by Oriel. Into the midst of this mass of confusion came the luckless Exeter; Pembroke were upon them in an instant, and Magdalen rushed into Pembroke, followed by Corpus, who managed to paddle past the debris and reach the winning post. Exeter and Balliol were each fined £5.

Taunt's Map and Guide to the Thames 1873 -

Oxford should be seen during the "Commemoration Week", which recurs annually in June: then, throughout the city, and on the river, pleasure is the order of the day, and everything wears holiday garb.
In the gay barges lining the beautiful banks of Christ Church Walk, enlivened by the varied costumes of many oarsmen, the river has, at Oxford, charms which it can boast nowhere else.

Oxford at Home
The Oxford Crew at Home

1874: BOATRACE - Cambridge by three and a half lengths. See Boatrace 1870s
1874: HEAD OF THE RIVER: University College

University Barge
The University Barge

1875: BOATRACE - Oxford by ten lengths. See Boatrace 1870s
1875: HEAD OF THE RIVER: University College
1875: U.S. professional oarsman Michael Davis patented the swivel oarlock -

The swivel rowlock caught on quickly in North America, but was slower to find a following in England. It wasn't until 1905, after the Belgians carried off a number of Henley pots while using swivels, did the British fully accept the device. Still, the fixed-pin rowlocks continued to be used in England where Leander Club won the Grand Challenge Cup in 1949 with fixed-pin rowlocks.

1876: BOATRACE - Cambridge by 5 lengths. See Boatrace 1870s
1876: HEAD OF THE RIVER: Brasenose

1877: BOATRACE - DEAD HEAT! See Boatrace 1870s
The finish judge was the Leander boatman, 'Honest John Phelps' who was somewhere on what he reckoned might have been the finishing line. (It was not marked by posts as yet). He was not helped by many spectators in boats around him. He is quoted as giving his verdict as - "Dead-heat to Oxford by five feet".
John Phelps was questioned as to what he meant -

For answer he placed the two palms of his hands together, and, moving them slightly backwards and forwards, said, “ They were going like this, sir; I couldn'’t separate them.

” 1877: HEAD OF THE RIVER: University College

1878: BOATRACE - Oxford by 10 lengths. See Boatrace 1870s
1878: HEAD OF THE RIVER: University College

1879: BOATRACE - Cambridge by 3 lengths. See Boatrace 1870s
1879: HEAD OF THE RIVER: Balliol