The 'myth' of Lochnagar

[dah]

 

"At Boiselle the earth heaved and flashed", he wrote, "a tremendous and magnificent column rose up into the sky. There was an ear-spitting roar, drowning all the guns flinging the machine sideways in the repercussing air. The earthly column rose, higher and higher to almost four thousand feet."

(Source: Cecil Lewis, RFC, flying above La Boiselle at 7.28am on 1st July, 1916)

 

This famous eyewitness account of ‘4000 ft’ columns (i.e. also including Y-sap) is re-quoted in many/most books about the first day of the Somme…...and I think it needs to be seriously challenged.

 

For reference/comparison, here's a picture of the seriously tall 'London Shard'

 

 

 

This building is 1,016 feet tall......seriously tall.....and yet only 25% of the 'quoted' height of the Lochnagar earth column. Can you look at the London skyline and realistically contemplate Lochnagar's earth column being 4 times higher than this?

(For those unfamiliar with the London skyline, an alternative reference height could be the Twin Towers in New York - which stood at 1.362 feet)

 

Lochnagar vs Hawthorn

We’re all familiar with Malins film of the plume of debris from the blowing of the Hawthorn Redoubt mine on 1st July, 1916. The height of that plume is estimated to be a few hundred feet (possibly no higher than the building immediately to the left of 'The Shard' in the photo above)

 

OK, Lochnagar (60k pounds of ammonal) had 50% more explosive charge than Hawthorn, but (remembering that geology and tunnel/chamber depth were almost identical) how does Lochnagar supposedly produce a column ten times higher than Hawthorn? It makes no sense.

 

I humbly suggest that the Cecil Lewis eyewitness account cannot be correct.

I can believe he was perhaps flying at 4,000 feet - and that his plane may have been buffeted by the force of the blast(s) but his estimated 'earthly column' height must surely be a gross exaggeration......and, if so, let's put a stop to the ongoing propagation of this 'myth'.

 

I'll start the bidding off at 400 feet.

 

Happy to hear alternative opinions/explanations

 

Regards,

 

David

[John_Hartley]

4 minutes ago, dah said:

I'll start the bidding off at 400 feet.

 

Happy to hear alternative opinions/explanations

 

Please provide some evidence for your suggestion.

 

That is, actual evidence rather than supposition - unless, of course, you happen to be an explosives expert in which case, I'll give some credence to the supposition.

[paulgranger]

The OP is a little adversarial, and I'd suggest that there's no 'myth' about the Lochnagar mine, but Lewis' description of the subsequent cloud height might stand scrutiny. After extensive research (i.e 5 minutes) on Google/Wikipedia, I've found that a 1 kiloton surface detonated atomic bomb would cause a cloud up to 10,000 feet. The Lochnagar mine was 60,000 lbs, or 30 kiloton, so quite capable of generating a cloud up to 30,000 feet, by extrapolation, though as it detonated at a depth of 60 feet, I would suppose this would have an effect on the height of the cloud, because the gases forming it would be compressed initially by the ground layers above. Still, based on the info, a cloud height of 4000 feet from Lochnagar would seem quite possible.

[The Scorer]

This may seem like a daft question (and my apologies if it is!), but how did Cecil Lewis know that he was flying at 4,000 feet?

 

Did his plane have such an accurate altimeter at that very early stage in the history of flight?

[paulgranger]

 

 

Don't know which aircraft this is, I just Googled WW1 aircraft, but there's definitely an altimeter.

[trajan]

18 minutes ago, The Scorer said:

This may seem like a daft question (and my apologies if it is!), but how did Cecil Lewis know that he was flying at 4,000 feet?

 

Did his plane have such an accurate altimeter at that very early stage in the history of flight?

 

Yes, altimeters were around.

[The Scorer]

2 minutes ago, trajan said:

 

Yes, altimeters were around.

 

Ah, okay, thanks.

[Jack Sheldon]

Although I only got O Level Arithmetic, I feel confident enough to state that 60,000 lbs is not equal to 30 kilotons. One kiloton is the term for the equivalent of 1,000 metric tons of TNT and 1 metric ton is 1,000 kg = 2,204.6 lbs. Thus the charge in the mine chamber weighed 27.216 metric tons. In other words, the charge was 29,972.784 metric tons less than you have suggested and it was amatol, rather than TNT. That said, amatol was chosen for its heaving ability due to the large amount of gas it produced, so it would certainly have sent debris flying high.  It was also an 'overcharged' mine, designed to produce an oversized crater rim which, it was hoped, would provide additional cover for the advancing troops.

 

Jack

[dah]

Cecil Lewis's account talks of an earth column.......not a cloud or plume of gas/smoke/heated air.

 

By way of further example, here is the earth column produced by the Hawthorn mine:

 

 

Let's say that column of earth (not smoke or gas) is 300 feet high, produced by 40k pounds of ammonal, detonated at the same depth and in the same geological conditions as the 60k pounds (i.e. 50% more energy) of ammonal at Lochnagar. 

 

SO, if it would help to re-phrase my original OP (be less adversarial)........if all other things are equal, how high do you think the above earth column would go with 60k of ammonal underneath it (rather than 40k)?

 

David

 

 

[paulgranger]

Oops. I blame the calculator.

[trajan]

Are those regular trees away to the left in the photograph? On the front line? Certainly not in front of it, but possible behind... So, the average full grown tree when cut for timber has a trunk around 10 m. or so... Now do the calculation... Alternatively, do a Ptolemy. We have the GB front line in the foreground (or is it the second - somebody will know!), we have the distance from that to the crater, and we can calculate the hypotenuse... 

[BullerTurner]

4 hours ago, Jack Sheldon said:

In other words, the charge was 29,972.784 metric tons less than you have suggested and it was amatol, rather than TNT. That said, amatol was chosen for its heaving ability due to the large amount of gas it produced, so it would certainly have sent debris flying high.

 

Jack

 

While we are checking maths, we might want to check chemistry or energetics!  The Lochnagar charge was Ammonal, a far less O2 balanced EM than Amatol or Schneiderite. It was also far more sensitive than Amatol which is possibly why the blind mine at Messines could be detonated so many years later by an electrical storm.

 

i know I'm a pedant.  Why on earth of you think I'm here??  It's certainly not to help!

[Jack Sheldon]

You are dead right. I do not know why I got that wrong. I have written about ammonal myself once or twice. That will teach me to be fussy about kilotons. Mind you, it does produce huge amounts of gas: hence its suitability for mining.

 

Jack

[BullerTurner]

You're alright mate! It is hump day and we all can start the run down to the weekend from here!  It did produce a lot of gas but nothing like Ammonal, which is why it was used in the Western Front mines despite reservations about sensitivity.  It cost less too...and our weapons are always sourced from the lowest bidder!!

[trajan]

And the trigonometry? Ptolemy did this in the whatever century before to calculate the distance from the earth to the sun (I think he was geocentric?). Now, you can establish the approximate base line; you can establish the approximate angle... C'mon, with those 'facts' the height can be calculated within a +/- error of at the very worse 50% easily - and it looks to be around, say, 1000 ?

[trajan]

1 hour ago, Muerrisch said:

The "trees"  at Hawthorn could easily be much much smaller than 10m. ... That alone makes any attempt at estimating debris height wildly suppositional. ... Given that the received wisdom is 4000 feet, I suggest it is up to those questioning that value to do the sums! 

 

 

First and foremost, let me make it clear that I have no 'stake' in the matter - nor did I pass GCE O level maths, so I'll not attempt the maths involved! But, I have done a lot of archaeological fieldwork and I am used to looking at landscapes and gauging distances and relative heights, etc. 

 

So, the distance between the photographer and Hawthorn Ridge was, let's say (I don't have detailed maps to hand) 400 metres? So what is the height of those trees on the ridge? Someone else can get the calculator out but they can't be much less than 10 m, I think - and clearly not 1 m.!!!

 

Also, the crater size. The eventual crater was about 100 metres across if I remember rightly? So, if that cloud of earth is proportional, and is spreading up from its origin, then it has a base of about 100-150 m. and can only be about 100 - 150 m high - or has that cloud come from a much wider crater in the order of 1,300 m diameter to give a 1,300 m. high cloud of debris? I confess I simply have no idea about the site or the effects of the bang, etc., but the ground spread of that blast is surely related to the resultant crater, and so about 100 - 150+ m wide at the base and so the 'cloud'...

 

Finally, the 'received wisdom' seems to go from a single observer in an aeroplane - or are there any other contemporary calculations / observations?

 

As I said, I have no stake either side on this issue, and can only make observations based on looking at landscapes - so I'll let others "... do the sums!" And point out (gently please!) any or all of the errors in my observations...

 

Trajan

Edited 13 July , 2016 by trajan
Spelling

[BullerTurner]

To be fair to poor old "Sagittarius Rising" he was very much an uneducated observer of the blast.  He may have mistaken a gaseous plume for the heave?  He might have misjudged the height of the heave relative to his own...I doubt his intent was to mislead or exaggerate.  After all, there you are tooling along in a string, canvas and wood box at 80 mph and suddenly a 27 kiloton blast goes off.  Your sangfroid is going to be tested...

[Dai Bach y Sowldiwr]

20 minutes ago, BullerTurner said:

there you are tooling along in a string, canvas and wood box at 80 mph and suddenly a 27 kiloton blast goes off

 

 

Or rather 27 tons.

(Post #8)

[BullerTurner]

Metric obviously Dai...

 

[Muerrisch]

We do not need to use trees to estimate height of Hawthorn heave.

 

Isaac Newton does the trick.

 

S = ut + ½ a t²

or, distance S [includes vertical distance] = initial velocity x time, plus ½ acceleration [in this case gravitational] x the square of time in motion.

 

At Hawthorn, the Malins film has a debris plume in the air for 8 seconds, assuming film speed is not badly distorted. [There is a dust and gases remainder afterwards, clearly composed of very light material which I have disregarded]. There are no relevant accelerations other than gravity [32 ft/sec/sec] ........ the accelerations due to the explosion are played out once the heave breaks the surface. Velocity at the top is zero.

From the top of the debris plume until it subsides to the new ground level is 4 seconds.

 

Thus :

 

Height = ½ x 32 x 16 feet, or 256 feet.

Hawthorn debris reached between 200 and 300 feet.

Homework for the GWF: Using the above, how long would a 4000 ft heave be in the air? [This relies on no comparison with any other crater, and needs no knowledge of explosives, subsoil or the name of the engineer’s mother.]

[Dai Bach y Sowldiwr]

1 hour ago, BullerTurner said:

Metric obviously Dai...

 

Hmmmmmm.

 

 

Ok, my contribution.

O level physics.

Time taken for an object to fall from a given height.

Assuming you ignore air resistance.

 

An object falling from the top of the plume -  400ft (122m), takes about 3.5 seconds to reach the ground.

An object falling from the top of the plume -  4000ft (1220m), takes about  11 seconds to reach the ground.

 

Figures more accurate for heavy clumps of rock or clay, but finer bits of soil and dust would take longer because of air resistance.

What does the film at Hawthorn  show, and is it playing at normal speed?

[Black Maria]

10 hours ago, BullerTurner said:

.  After all, there you are tooling along in a string, canvas and wood box at 80 mph and suddenly a 27 kiloton blast goes off.  Your sangfroid is going to be tested...

Cecil Lewis was told beforehand that the mines were to be blown and his orders were to ' Keep clear of La Boisselle'. He turned South and " sailed down" to

watch the mines go off . I add this just in case anyone reading the above comment thought that Cecil was caught unaware by the mines being detonated. I

don't know how close he was to the explosion but he states that his machine was flung sideways by the "repercussing air".

[Callard]

I would suspect that he would have seen the shock wave which would have travelled far further than the soil element, this would have been accompanied by super heated gas. This may well have given the impression of the blast being much higher than the soil and rock content would have reached. Just a thought.

[trajan]

What I find somewhat astounding is that until dah's OP nobody seems to have thought to do an independent check!

[Stebie9173]

Edited out (question re crater size compared to column width already asked above)

Edited 14 July , 2016 by Stebie9173