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iments at various distances, to wit, from one mile to twelve miles or more—And for the purpose of measuring the time, I have a very accurate portable instrument which is automatic and furnished with a pendulum vibrating half seconds.
That I might proceed the more safely I proposed to myself the following questions to be discussed:
1. How great is the space which sound traverses in a sec- ond or other interval of time.
2. Whether a gun fired towards an observer sends the sound in the same interval of time as when it is fired in the opposite direction.
3. Whether in every state of the atmosphere when the mercury is rising or falling in the barometer sounds traverse the same space in the same interval of time.
4. Whether sounds are propagated with greater velocity by day than by night.
5. Whether a favoring wind accelerates sound and an opposing wind retards it – that is, whether winds affect sound at all, and if so, in what manner.
6. Whether sound is propagated with a greater velocity in a tranquil atmosphere than when the wind is blowing.
7. Whether a strong wind blowing crosswise accelerates or retards the velocity of sound.
8. Whether sounds have the same velocity in summer and winter by day and by night.
9. Whether sounds have the same velocity in snowy and in clear weather.
10. Whether a great and a small sound have the same velocity.
11. Whether at all [angular] elevations, viz: horizontal, ten degrees, 25 degrees, and so on to 90 degrees, the sound of a gun- shot strikes the ear of an observer at the same interval of time.
12. Whether sounds of all kinds, i.e., of guns, bells, ham- mers etc., have the same velocity.
13. Whether charges of powder varying in strength pro- duce a variation in the velocity of sound.
14. Whether on the tops of high mountains and in val- leys, that is, whether in the highest and lowest parts of the atmosphere, sounds travel the same space in the same inter- val of time.
15. Whether an upward and a downward sound have the same velocity, that is, whether it descends from the top of a mountain to its base at the same rate as it ascends from the base to the top.
16. Whether sound is propagated in the beginning with greater velocity and in the end with less, as occurs in many other violent motions.
17. Or whether, on the contrary, it is uniform—that is, whether or not it is propagated half the space in half the time, one fourth of the space in one fourth of the time, etc.
18. Whether in all regions northern or southern—in England, France, Italy, Germany, etc.—sounds have the same velocity. [This question seems particularly strange but Derham’s discussion in Section 9 gives the background.]
19. Whether sound passes from place to place in a straight line, (i.e., by the shortest path), or according to the [irregular] superficies of the intervening land.
For the settlement of these questions I asked some kind friends of mine, (whose favors I here most gratefully acknowledge) to fire muskets from towers and other high places at a distance of one, two, three and even as far as eight miles, (which I have found to be the greatest distance at which I could hear the sound of a gun in these parts, covered as they are with woods, etc.). These musket shots were of great use to me. But what especially answered my purpose were the cannon (Sakers [Welling’s note: The saker is a species of small ordnance.]) which are used at Blackheath in training the raw recruits who are to serve the artillery of our most illustrious queen. I could see the flash and hear the report of these cannons from the tower of my church; some- times also I made use of a telescope. And hence I have devot- ed myself with all care and diligence to the observations of these cannons since February, 1704.
After a few observations made in the midst of their dis- charges, I took measures for making a certain particular experiment through the courtesy of the late Baron Granville then the Governor of London Tower and of other eminent men who on that Tower serve the interests of the royal artillery, (and whose favors I here most gratefully recog- nize). Two cannon (Sakers) were placed side by side one with its mouth towards me the other with its mouth reversed. These two cannon on the 13th February 1704 were fired every half hour from 6 o’clock P.M. to midnight, while a gentle breeze was blowing directly against the sound. The interval between the flash of each gun (which flash I could see with the naked eye) and the arrival of the sound was always about 120 or 122 half seconds – I have said 120 or 122, since the sound came to me duplicated – that is the first sound came within 120 half seconds (which was a weaker sound) and the second within 122 half sec- onds (which was a stronger sound) and in the same manner, through the whole time of the observation, the crash of each cannon came in a duplicated form.
This reduplication of sound seems to me an echo, reverberated, as I think, from the watermill at Blackheath, or from the houses situated in its vicinity. [London Tower is just slightly south of west from Derham’s church at Upminster; Blackheath is west southwest from Upminster and about 3/4 as far as London. The azimuth angle to Blackheath is more southerly than to London but not by more than 30 degrees.] I have no reason for doubting about this point except the contrary opinion of a learned friend and sagacious philosopher who believes that there is no echo to be heard save that produced by objects reflecting sound near the observer, instead of its being produced by those near the sounding body—or other distant objects.
The next disquisition, therefore, will be,
2. Concerning sounds reverberated from a distance, or the distant echo.
Perhaps this disquisition will be considered a digression but since it pertains to the subject of sound I hope that a few observations on this subject will not be ungrateful to ingen- ious minds.
And in the first place I believe that this [the fact that the
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