SEA LEVEL ISOBARS? By J. M. Walker University of Wales Institute of Science and Technology

That pressure decreases with increasing altitude is familiar enoug 1 to most people; it is not often, though, that one sees such a striking exam1 le of this principle as is provided by the barograph trace shown in Fig. I. I urther, it is customary to regard a ship as a vessel which, necessarily, stays a t sea- level, but even the mariner, conversant though he is with techniques for correcting his barometer to sea-level, is likely to overlook the fact that, when he journeys up the St Lawrence Seaway to the Great Lakes of North America, he is no longer a t sea level.

The barograph trace shown resulted from the passage of the Toronto City, 5 705 tons, of Bristol City Lines, from Lake Ontario to Lake Erie through the eight locks of the Welland Canal, on 26 August 1959. The pressure re- ductions at seven of the locks are visible clearly, but the ship suffered a delay of 9 hours before ascending to the level of Lake Erie, thus disguising the pressure effect a t the last lock. The average upbound passage through the canal takes about 11 hours, whereas the down journey is faster, about 8 hours, because the locks empty more quickly than they fill.

The Welland Canal is 28 miles long and provides a by-pass to the Niagara Falls. The ascent of 326 ft from Lake Ontario, itself 246 ft above mean sea level, to Lake Erie is equivalent to a pressure drop of about 10 or 11 mb (using the I m b ~ 3 0 ft rule of thumb) and it can be seen that the observed pressure- decrease, which, it should be remembered, is superimposed upon the larger- scale pressure-pattern variations, is indeed of that magnitude.

(Barograph trace by courtesy of Bristol City Lines.)

Fig. I . ,Barogram from a ship traversing the locks of the Welland Canal, during which the ship ascends 326 f t as it goes from Lake Ontario to Lake Erie