The noise (cannon fire) angle BMCS brings up relates to the explanation that IMHO seems to best fit the largest number of fragmentary bits comprising the story as
it was specifically related in writing in the September 1767 letter.
Here are some key points from the letter's own text and / or background facts, some of which I tracked down from local historical accounts:
- The earliest observations are related as occurring "on the water of Isla" at Coupar Angus.
- The final observation is related as occurring at or "above" Blairgowrie.
- There's little or no chance any individual observer witnessed the entire incident from start to finish (including the cart, house, and rider bits).
- None of the 3 auxiliary effects (cart, house, and rider) occurred upon the river(s) per se, but on land (i.e., the house site, the high road, and wherever the cart was sitting).
- We are given no clues as to where these 3 auxiliary effects occurred, meaning they could well have occurred anywhere in the local countryside between or surrounding Coupar Angus and Blairgowrie. They could have occurred in close proximity to each other or at widely separated locations.
- Nowhere does the text claim these three auxiliary effects resulted from direct impacts with the object, and none of these three incidents are claimed to have been witnessed as they occurred (save for the thrown rider's first-person testimony, such as it's related ... ).
- With the possible exception of the rider's experience, the cart / house / bridge incidents' descriptions could well represent
post hoc explanations rather than realtime observations.
- The object resolved or emerged from smoke.
- The object was described as "pyramidal" - implying a polyhedral or irregular shape with apparent vertices.
- The object was "luminous".[1]
- The object "rolled". That's it; that's all. Nowhere does the text claim it "rolled upon" (the ground or the rivers' surfaces).
- There was no (finished; functional) bridge spanning the Ericht at Blairgowrie until 1777 (ten years later). This was apparently the "original bridge" repaired in the first decade of the 1800's owing to flood damage and completely replaced when destroyed by floods in the 1840's. The few references I can find for this "original bridge" suggest it was a rubble-filled arched span similar to the other substantial bridges in the area.
- This "original bridge" was built by local subscription prior to the period during which there was a widespread military bridge building campaign in the region.
- This "original bridge" would have been in a quite rudimentary stage of development ten years prior to its completion.
- It's unclear whether the "immediate disappearance" upon damaging the bridge under construction means (a) the object spontaneously vanished versus (b) the object simply traveled onward until lost to view.
- The letter containing what was probably a compiled collection of observations and claims was dated 8 September and sent from Edinburgh. Considering the time necessary to collect the story's tidbits, convey them somehow from Perthshire to Edinburgh, and finally write the letter it's anybody's guess when the incident actually occurred. It would probably have been days - or conceivably even weeks - prior to 8 September.
[1] This clearly contradicts UFO accounts referring to the object as "silver" and Sid's (admittedly creative) reinterpretation of it as being "mirror-like." "Luminous" does not mean "shiny" - it means "shining" (as in glowing, giving off light, incandescent, self-illuminated).
This story has sometimes been treated as a USO incident, as if the object arose from the River Isla and traveled on or along the Isla and Ericht watercourses. This surface level elevation and riverine path are consistent with the letter's opening statement the phenomenon was "observed on the water of Isla." This phrasing has been pretty consistently treated as if it meant the phenomenon emerged / appeared and traveled within, upon, or in very close proximity to the rivers.
However, there's another defensible interpretation of the phrase "observed on the water of Isla" - i.e., that it refers to the earliest observer's / observers' position(s) and vantage point(s) rather than the location of the object itself. "On the water" could have meant the earliest observers were fishing on the river. In a more figurative sense, it could have meant they were at or adjacent to the river.
I suspect the letter describes a meteor associated with the recurring Perseid, Kappa Cygnid, or Aurigid showers. It was first observed from somewhere adjacent to the River Isla near Coupar Angus, and was subsequently viewed as tracking northwestward as if following the River Ericht. The descending meteor had an initial smoky / dusty trail from which an incandescent irregular shape emerged and was observed to tumble in flight.
I suspect the cart, house, rider, and bridge effects resulted from a sonic boom or airborne concussive blast caused by the meteor breaking up. (I've witnessed such booms; they do indeed resemble the cannon fire BMCS mentioned.) For example:
- The house half-collapsed;
- The cart was shaken into rolling a short distance; and ...
- The fragile still-being-constructed proto-arch of the bridge simply collapsed [2] in response to an abrupt vibrational shock.
The rider may have been thrown in response to this hypothetical boom, but it could also have resulted from the sight of a prominent meteor / bolide passing overhead (e.g., spooking his horse).
[2] I suspect the letter's use of the term "undermined" is a red herring representing the reasonable impression one may retain from hearing about something collapsing rather than being visibly impacted - i.e., it must have been destabilized from below.
Finally ... There's indeed a remarkable aspect to this otherwise mundane hypothetical interpretation ... It would mean the 1767 meteor's fall bore a close resemblance to a proven meteor (with meteorites recovered) following a very similar track 150 years later in 1917:
https://www.nms.ac.uk/explore-our-collections/stories/natural-world/strathmore-meteorite/