'A thousand feet, sir? We can rely on that?'

'We can't rely on anything. Mr Dalrymple at the Hydrographic Office admitted he knew nothing much about it - he just warned me not to hit Martin Vaz, which is either a tiny island or a reef of rocks a day's sailing from it.'

'Once we're through the Doldrums, we'll get a lift to the westward from the current,' Southwick commented. 'But just think of it, once we make a landfall we go on shore to plant potatoes ... I hate gardening,' he admitted, 'but it'll make a change to carry a spade and not a sword!'

The Doldrums had been empty days when the Calypso sat dead on the water, the heat haze merging sea, horizon and sky into what seemed to Wilkins a pool of molten copper. It was a time when he wanted to paint, wanted to capture on canvas the sense of the empty vastness of the ocean when there was no wind, where the sails were furled on the yards because there was no point in leaving them chafing against masts and rigging with every movement of the ship. Some days there was a slight swell, and Captain Ramage said it was caused by some distant storm, probably several thousands of miles away. He wanted to put it on canvas, but the sun was too hot. Even under the awning stretched across the quarterdeck it was an oven which sapped everyone's energy. Tempers were fraying and the sentry on the scuttlebutt watched closely as a man dropped in the dipper and took a drink.

The sheer stark simplicity of the life fascinated Wilkins. The intense heat, the lack of wind, and the fact that the Calypso was taking twice as long as expected to get through the Doldrums meant the men were twice as thirsty but had only half the water. It was interesting that the men had a basic ration, but in addition some extra was put into a butt each day and this was left by the mainmast with a Marine sentry guarding it.

And there was a dipper, a cylindrical, open-topped container, the diameter of a broom handle and about four inches long. There was a hole on each side of the top through which the line threaded, and a man was allowed to drop the dipper down through the bunghole and draw out as much water as it would hold. But because when there was a water shortage the butt was stowed on its side with chocks, the dipper usually tilted before it could fill completely. And the Marine sentry made sure that it was 'one man, one dip'.

Still, Wilkins had made up his mind about the colours, sketched in the outlines on the canvas with charcoal, and was ready when the first teasing but cooling puffs of wind had come. First of all there had been an excited hail from a lookout at the masthead - a man perched in what looked like an open-sided tent with strips of canvas to protect him from the rays of the sun. 'Wind shadow on the larboard quarter!' he had shouted. A couple of minutes later he reported it was approaching, but, just as suddenly, it vanished. Five minutes later another, also on the larboard quarter, reached the ship, a wind shadow that danced across the surface of the sea like a swarm of gnats on the edge of a pond. Suddenly they all had a teasing breath of cool air, but then it was gone.

Yet Mr Ramage was quite confident the wind would set in: topmen swarmed aloft to drop the sails - 'let fall', rather. And by then more wind shadows were being reported, the men becoming excited, and he was hurriedly mixing paints on his palette, the sudden breeze blowing away the lethargy. Now they were at last in the southeast trade winds which Southwick said started off down towards the Cape of Good Hope.

Crossing the Equator a few days later was best forgotten as far as Wilkins was concerned: Neptune had dozens of victims because the Calypso had spent most of her time in the Caribbean or Mediterranean, and few men had crossed the Line. So the unlucky ones were given stiff 'tonics' of soap and water, shaved, ducked and five men, who had objected violently, were ordered by King Neptune to be tarred and feathered. Three others had their faces and backsides given a liberal coating of black gun lacquer.

Wilkins found it hard to get used to the sun's position. It was sufficiently late in the year and they were far enough south for the sun to be almost vertically overhead, so his shadow at noon was tiny, extending only a few inches from his feet, as though he was standing in a small puddle. Flying fish skimming just above the waves like great dragonflies had been commonplace for a long time and although the Calypso was now almost midway between West Africa and South America, he was surprised by how many sea birds they saw. He had painted some of them, putting the date and position on the back of the canvas. He enjoyed painting birds in flight because it gave him good practice at painting the sea - surely the most challenging of all subjects. It was never the same, varying with the wind, cloud, sun, or rain, and, according to the captain, with the depth of the ocean and the latitude. At Trinidade, their destination, the captain promised that if there were reefs, he would see three or four different colours in as many hundreds of yards. For the moment, though, everyone was relieved that they were now in the southeast trades.

Now Southwick, legs astride and balancing himself against the gentle roll, flipped down one more shade of his quadrant because he found the sun bright, once again 'brought the sun down to the horizon', rocked the quadrant slightly to make sure that the lower edge of the sun was precisely on the horizon, made a slight adjustment and a moment later saw the sun had moved. He read the figures on the ivory scale of the quadrant. The ritual of the noon sight was, as far as the sun was concerned, now over: he had measured the highest angle that it made with the horizon, and that was that: only the angle mattered, not the time: if he had measured its highest angle, then that was the angle at noon local time, and he did not have to bother to turn a half-minute glass, bellow at Orsini to note the chronometer . . . Now he had to apply some corrections, add or subtract figures from the almanac, and the answer would be the Calypso's latitude. It was the simplest thing to do in celestial navigation; it was how the navigators from the oldest times crossed oceans - they knew the latitude of their destination and sailed along it until they arrived. The only danger was running into the land at night. Longitude was a different problem; without an accurate chronometer there was no way of being sure of one's exact distance east or west of the Greenwich meridian.

Young Orsini was working out his answer using the top of the binnacle box. Kenton and Martin were sitting on the breeches of guns. Southwick could see Mr Ramage walking up and down on the windward side of the quarterdeck, having his spell of exercise before his meal. And waiting to hear the latitude ... Normally Mr Ramage left the navigation to him, but for the last three days he had been taking a close interest. The reason was not hard to guess - the Calypso's latitude and longitude were almost the same as the figures they had been given for Trinidade.

In fact, according to Southwick's reckoning, they were within a hundred miles of it. Allowing for the chronometerbeing a bit out, he was sure that putting the point of a pair of compasses down on Trinidade and drawing a circle with a radius of fifty miles would enclose the Calypso, but therewas a high haze, so it was impossible to guess whether they could see ten miles or sixty.

This was always the difficult time when making a landfall: did one set more canvas to increase speed in the hope of sighting land before nightfall, or go slowly and cautiously and hope to sight it at dawn? Martin Vaz should be on the larboard bow and Trinidade dead ahead. If one left Martin Vaz too far to larboard - thus making absolutely certain of not hitting it - there was a risk of passing Trinidade out of sight to larboard. The life of a master in the Royal Navy, Southwick thought to himself, could be summed up by that situation: trying to find one rock in the middle of an ocean without hitting another . . .