Sound: A Course of Eight Lectures

...to the mixed currents of water vapor and air, neglecting differences in temperature. I never doubted that differences in temperature, when they occur, are an effective cause of acoustic opacity Acoustic opacity refers to the atmosphere's resistance to the transmission of sound.. In fact, this research provides the first experimental demonstration of sound reflecting off the air due to this cause. I will not attempt to state the exact relative strength of temperature differences versus differences due to water vapor in the cases under consideration. However, since both are active factors, I have referred to them together in Chapter 7 as being responsible for producing "acoustic clouds" Tyndall's term for invisible patches of air that reflect or block sound waves, similar to how visible clouds block light.. It is to these clouds that the stoppage of sound in the atmosphere is for the most part due.

After the full investigation was published, however, another criticism appeared. Because of its source, I want to address it with full respect and attention. This criticism first reached me through the columns of an American newspaper. It claimed that neither differences in the amount of water vapor nor differences in temperature are effective causes of acoustic opacity. At a meeting of the Philosophical Society of Washington, an emphatic opinion was reportedly expressed that I was wrong to attribute the opacity of the atmosphere to its flocculence original: "flocculence"; here meaning a patchy or uneven consistency in the air that causes sound to scatter.. They argued that the truly effective cause was refraction Refraction is the bending of waves, such as sound or light, as they pass from one medium to another of different density.. This view seemed so clearly mistaken to me that, for a time, I assumed the newspaper report was inaccurate.

Recently, however, I received the "Report of the United States Lighthouse Board for 1874," which confirms the account I just mentioned. A brief reference to this Report will be enough here. Major