The Boston Journal states that McKay, of the clipper Sovereign of the Seas, built in Boston, previous to sailing from this city, (N. Y.,) tor San Francisco, in August last, addressed a latter to Lieut. Maury, of the National Observatory at Washington, requesting a copy of the fourth edition of his Sailing Directions, for the use of the voyage. Lieut. Maury answered the letter, stating that if Capt. McKay would follow the directions laid down, the Sovereign of the Seas would be able to cross the Equator in the Pacific on or before the 25th day of October, and would reach San Francisco in one hundred and three days. The Sovereign of the Seas crossed the line only 14 hours behind the predicted time, and dropped anchor in the harbor of San Francisco in one hundred and three days and two hours after leaving New York. This prediction on a voyage of 17.000 miles, is a forcible illustration of th benefits of modern scientific research.