sometimes to make sound audible against the wind. Thus Henry shows that a sound moving against the wind, inaudible to the ear on the deck of a vessel, could be heard at the masthead. Reynolds's experiments even more conclusively demonstrate the bending of the wave front downward as a rule when moving with the wind and upward when moving against the wind. The accompanying photographs, Plate V, figs. 27 and 28, show air strata moving with varying velocities. As a rule the upper currents have the greater velocity, but not infrequently this condition is reversed. In such cases audibility should be favored even by an opposing wind, and this is sometimes found to be the case. Thus far we have alluded only to the refraction of the wave fronts due to varying air velocities; but the varying temperatures of the different air masses will also affect the relative audibility. Reynolds instances a marked case, where, owing to a thorough cooling of the lower air strata, and presumably a marked inverted temperature gradient, the audibility was excellent, the sound being refracted downward, and all objects "looming," as it were. It is even possible to work out the retardation or acceleration of the wave front with the degree of variation in temperature. Finally, it may be that the temperature and the air motion may act together to refract downward the sound wave, and it may also happen that the one influence may oppose the other. Thus Reynolds gives an example where, with a heavy dew on the ground, sound could be heard equally well against a light wind as with the wind "Showing that the upward refraction by the wind was completely counteracted by the downward refraction from the diminution of temperature. This was observed not to be the case when cloudiness at night prevented terrestrial radiation." (Proc. R. S., 1874.) The presence of large quantities of condensed water vapor brings us to the question of refracting surfaces and the reverberation of the sound rather than its velocity. When a sound wave travels over a perfectly smooth surface, such as a glassy sea, or a sharply outlined plane of condensation, the intensity of the sound does not diminish with the usual rapidity. In discussing the propagation of sound in whispering galleries, Rayleigh" shows that the abnormal loudness is not confined to a point diametrically opposite that occupied by the speaker, but that there is a bending or clinging of the sound waves to the surface of the concave wall. Sonorous vibrations at fog surfaces and cloud surfaces may behave in a somewhat similar way, and it is probable that the curvature of the surface is not of as great importance as the comparative smoothness of the surface. Probably the roll of thunder is an excellent illustration of continued reverberation at cloud surfaces. Any memoir upon the fog of San Francisco Bay would be incomplete without a reference to one of the most remarkable of marine accidents. On the morning of Friday, February 22, 1901, the Pacific Mail steamship Rio de Janeiro ran upon the Fort Point Reef during a fog. Within fifteen or twenty minutes from the time of striking the vessel sank, and of the 210 persons on board 130 were lost. Another statement, purporting to be official, makes the total number aboard 207 and the lost 127. The following facts are obtained chiefly from the statements of Pilot F. W. Jordan. The ship's master, Capt. William Ward, went down with the vessel. The pilot boarded the Rio de Janeiro in the vicinity of the 9-fathom buoy, near the bar buoy, and anchored in 13 fathoms at a little before 6 o'clock Thursday night, February 21. The weather being foggy, the ship remained at anchor till about 4 a. m., one hundred and twentieth meridian time, when the fog lifted and the Cliff House light could be seen, but not the Point Bonita light. Preparations were in progress to steam into the harbor, when a dense fog came out from the Golden Gate, obscuring everything. There is some difference of testimony as to whether the captain or the pilot gave the orders to proceed in the fog, but the vessel was started on a northeast course with Lime Point straight ahead, steering by the whistle. The pilot expected to get an echo of the ship's whistle from Point Diablo, but heard none. The course was changed north-northeast with the intention of running close to Lime Point. The vessel was not moving at full speed and was subject to a strong cross current, which, apparently acting at right angles to the length of the vessel, carried the ship to the south, far out of the proper course. The first officer was standing on the starboard side listening for the Fort Point bell. No soundings, however, were taken. The vessel struck a short distance to the southwest of the Fort Point light. At the moment of striking the pilot saw the white flash at Fort Point and heard the Fort Point bell. The pilot had had eleven years' experience in the harbor and was considered one of the most capable pilots in San Francisco. He had never previously met with an accident. There appears to be no doubt of the existence of the strong cross current, inasmuch as other vessels entering a Theory of Sound, vol. 2, sec. 287. the harbor about the same time on the same morning came near going on the rocks. The Pacific Mail Steamship Company has a rule that vessels must not enter or leave a harbor when fog prevails. On the morning in question the fog lifted for a few moments and then settled down again; but by 9 a. m., local time, the fog had dissipated. The rest of the day was clear and balmy and the water as smooth as a mill pond. The vessel struck about 5.30 a. m., with the pilot and captain on the bridge, the first officer on the starboard side of the bridge listening for the bell, and the second officer at the telegraph. When the vessel struck, the captain blew the danger whistle, a long blast. Ordinary fog blasts (long enough to count six or seven) had been blown previously. The details of the accident, particularly with reference to the whistles, the course steered, and the motion of the current, are given, because from such evidence as can be obtained at this writing it appears that even after the vessel struck the sound of the whistle was not heard plainly at Fort Point, not more than half a mile distant, where a lookout of the life-saving station was on duty and where a life-saving crew could have hurried to the rescue and probably reached the ship within five minutes, without doubt saving many of those whose lives were lost. There was also a sentry walking post within a short distance of the lookout. It is stated that some soldiers heard voices and also a whistle, but the evidence is very conflicting, and it seems improbable that if the long danger blast was clearly heard it should pass without notice and subsequent action. The Weather Bureau records show that about the time of the accident a mongrel tule fog prevailed over the Bay of San Francisco. At Mount Tamalpais the weather was clear, with a wind of 13 miles per hour from the northwest. At San Francisco dense fog prevailed, with little, if any, wind. The wind vane at the Mills Building indicated a southwest wind blowing about 1 mile per hour. From 1 a. m. to 6 a. m. but 9 miles of wind were recorded. When all is said and done it appears that the fog was the prime factor in causing the loss of the vessel. Owing to the aberration of the sound waves in the fog the pilot was unable to hear the fog signals from either Point Bonita or Lime Point to the north, or the tolling of the bell at Fort Point to the east and north. It has not, however, been shown that the bell was certainly ringing. The Lime Point whistle has great penetrative power. The fog bell at Fort Point is 40 feet above the water, and should be heard for at least a mile. It is supposed to be struck every ten seconds. It is a strange fact that in a paper upon the Fogs and Fog Signals of the Pacific Coast, by Ferdinand Lee Clark, published in 1888, there should occur this statement concerning the fog bell at Fort Point: In point of fact it is said to be hardly ever heard except when too late to be of use. * If mariners depended upon its sound to tell them how near they were to the point, they would generally have no time after hearing it to clear the danger. * * The loss of the Rio de Janeiro proves that the bell at Fort Point in its present position is sometimes of little value. The temperature at the time of the accident was 50° F. at sea level and 52° at a height of 2,500 feet. The thickness of the fog probably did not exceed a few hundred feet; and, as indicated above, it was a land fog rather than a sea fog. As a general thing the reflected sounds from Point Bonita and Lime Point are heard better on the south side of the channel. On the accompanying rough sketch of the channel the lines of natural reflection are drawn and also the zones of inaudibility. It is not difficult to account for the failure of the echo of the ship's whistle from some portion of the northern shore, as the distance of the vessel from Point Diablo was too great. The sound waves from the fog whistle at Lime Point, however, should have been heard, and as the moderate southwest wind would tend to cause a deflection of the sound wave upward it is possible that while the sound was inaudible on the deck of the vessel it might have been heard by a lookout at the masthead. The catastrophe furnishes a remarkable illustration of the utter helplessness of a vessel in fog, despite lights and fog whistles. It would seem that under such conditions nothing short of some method of fog dissipation will suffice. It has occurred to the writer, although the suggestion may prove of no value in practice, that if a strong sound had been made under water by some automatic contrivance at either Lime Point or Point Diablo, and the Rio de Janeiro been provided with some suitable device rendering audible the sound wave through the water, the strong cross current would have facilitated the passage of the sound and a zone of audibility would have been established in the water, while in the atmosphere above the fog signals would have been inaudible. Scale. Inaudio Supposed Course of PM SS Rio de Jane Probable place of Wred Bell Fort Point Light Pt Cuvallo Inaudible PRESIDIO US Military Reserve USLSST DU.SL.SS FIG. 31.-Probable conditions at time of wreck, February 22, 1901. The accompanying photographs, Plate VI, are submitted to show that to a certain degree the captain and the pilot were justified in assuming that they might soon run into areas free from fog. As a matter of fact on the day in question the fog soon disappeared and a delay of perhaps two hours would have prevented the accident. It should not be forgotten, however, that the captain was unwilling to enter the harbor during the fog Thursday night, and that the vessel remained at anchor for a period of nearly twelve hours and was thereby exposed in a large degree to the danger of collision. It is sometimes stated that thunderstorms are exceedingly rare in California and that lightning is almost unknown along the coast. And it is generally believed that the Pacific coast, or at least the, southern half of it, is a region free from thunderstorms and the damage by lightning is practically an unknown quantity. In the eastern part of the United States considerable damage is done by thunderstorms between the months of April and September. In California thunderstorms may occur during any part of the year. In a discussion of 356 reports of thunderstorms in California from July, 1895, to August, 1896, we found that there were 3 dates in July on which storms occurred, 6 in August, 8 in September, 10 in October, none in November, 3 in December, 5 in January, 2 in February, 3 in March, 9 in April, 15 in May, 6 in June, 22 in July, and 17 in August. Some of the storms covered very large areas and were quite generally reported, such, for example, as October 14-15, 1895; May 29 and August 28-29, 1896. Examining certain marine reports, it appears that on January 25, 1896, thunderstorms prevailed in the Pacific Ocean, and it is not surprising to find that a day or two later thunderstorms were reported in California. These storms apparently moved inland from the ocean. There is another class of storms, however, apparently connected with the general low-pressure movements from the southwest. A condition favorable for thunderstorms in the valley of the Colorado in July and August is frequently followed within ten or twenty hours by thunderstorms along the Sierra. The following table shows the distribution of thunderstorms in California during the year of 1895-96: THUNDERSTORMS. 2,750 2,825 Guinda. 5,000 293 |
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