Page 35 - January 2009

these experiments, I nevertheless wished afterwards to try them over again at greater distances, but the opportunity was wanting. I leave this matter therefore to be better tested by others.
7. Concerning the uniformity of the velocity of sound.
The next observation was concerning the uniformity of the velocity of sound. I have found this to be the same as the illus- trious Accademia del Cimento has already defined. That is, sounds traverse half the space in half the interval of time; one fourth the space in one fourth the interval of time and so on. This fact will be plain from the examples in the following table:
The distances of the places marked in the table from Upminster (where I made my observations) I measured with as much accuracy as I could either by the surveyor’s rod or by trigonometrical art. [There are two small errors in this table. The actual distance from the Lower Church to the Church at Upminster is 3.10 miles, not 3.0 miles and the distance from the South Weal Church is 4.64 miles.] And from the great consonance between the distances measured in this way and by the velocity of sound, the excellence of my instruments, the truthfulness of my calculations and observations is set in a clear light. For the difference between the distances ascer- tained by measurement, and taken from sound, either disap- pears entirely, or is that of only a very few hundredths of
parts, unless where there may have been a favoring wind (the case of south Weal Church being excepted, concerning which hereafter). So, too, in the observations made from the churches at Dagenham, Warley, Thorndon, and Barking, the distances marked by sound seemed a little shorter because the wind accelerated the sound. But in working up this col- umn of distances by sound, I have allowed nothing on account of the acceleration of the winds, but have simply divided the number of the vibrations, or half seconds, by 9 1/4 or 9.25 (the number of half seconds in which sound trav- erses a mile.)
The equible rate of the motion of sound is also manifest from this table, as will appear from a comparison of the vibrations and the distances, or from the column of the dis- tances alone as derives from sound.
But that nothing might be wanting in confirmation of these facts I made a journey to Foulness Sands on our Essex Coast. These Sands, washed and covered by the daily tide of the sea, make a great and smooth plain for many miles. Upon this plain I measured off only six miles, because neither the tide nor my leisure permitted that I should measure a greater distance. At the end of almost each mile I made experiments by the firing of muskets, not without great peril to my life from the influence of the sea and the darkness of the night. From these experiments I found that all my former observa-
34 Acoustics Today, January 2009