From Archibald McLachlan   New Years Day

New Years Day

D^r Tyndall¹

Procrastination – just as usual, and what other way should it be – were I to be punctual you might suppose something was wrong. But its all right, just what might be expected – And now allow me to wish you a good New Year – with us here of the O.S.² it is dull enough god knows. I heard one fellow express himself ‘If I was at home I would think it a Disgrāse to spind³ the day this way’. If I don’t tell you now I will forget it – I had two letters from Bob Bloomfield⁴ he is in Scotland on some railway between Paisley, Dumfries, & Glasgow⁵ – he tells me that a C.E.⁶ an old acquaintance of mine has been asking him concerning me. Bob says, ‘I put in a good word for you’ – so peradventure I may fall in for a good job – Two extra causes has delayed me in writing this letter – One is, I expected to get a peep at all the Tidal observations. I am disappointed – But I have got what is much better, the opinion of a learned man⁷ who calculated and arranged the whole thing. and secondly I expected to steal a copy of the Tide Wave,⁸ I have been able to pilfer only one Trace of one Station, another Trace I am sure I can get during this week, and, this, or the next week I am told I can manage to trace of the ‘Shannon Wave’.⁹ I look on this wave as the most important & interesting –

– A great wave follows the course of the Moon, Sir Isaac Newton and others calculate the moons mean wave at 9 feet¹⁰ – The Suns wave at 2 feet – This wave forms what we call the tide – there are 2 waves, one due to the moon, and one to the Sun – the Moons wave follows after the moon, no matter where the Sun is – the Suns wave follows the Sun no matter where the moon is – so the 2 waves seldom coincide – Suppose the Sun & moon to be at right angles, or, suppose the moon to be over head, and the sun at that moment to be just setting – we will at this moment (or nearly so) have high water of neap Tide¹¹ – the moon loses ¾ of an hour every day – the Sun gains upon her ¾ of an hour every day – till at last he comes up with her – If he is gaining ¾ of an hour on the moon, of course his tide of 2 feet is catching or following her tide of 9 feet – & when the Sun has come up with the moon (as is the case in the eclipse of the Sun) the tide which followed the Sun will be on the top of the moons tide, the 2 waves are then run into one wave and form what we call Spring Tides¹² – The height of those waves will vary according to the season – or the distance of the earth from the Sun – The true form of the Tidewave cannot be determined, except by measurements taken in the open, wide Ocean, free from the obstruction of headlands Continents &c – The object of our O.S. measurements is of a local nature, to determine the mean rise or fall of the Tide at any given place, and thus fix a Datum line for our O.S. levels – round the coast of Ireland there are, or were 22 Tide gauges (Poles or Boxes, with Feet and inches marked on them and sunk or placed the most convenient place) at each gauge a man was placed with a proper book, and chronometer – the observations were made at all the Stations on the same days – they all started at the same moment and ended at the same moment – each man entered in the book the height of the tide on the gauge & that was all he had to do – he entered the height every 6 minutes –

Thus!	Hour-M	feet	in
Augt 25. – – A.M.	6.0	14 –	4
	6.6	14 –	0
	6.12	13 –	9
	6.18	13 –	5
	6.24	13 –	0
	&c	&c

Now you will see with ½ an eye that unless the observations are regularly entered, the whole is useless – suppose the observer gets tired of watching – or – falls asleep – and wakens after 2 hours sleep! He could by having the first half hours observations carefully made he could construct or fill up the book by allowing a fall or rise of Tide of 4 inches for every 6 minutes!! this may, and has been done to an extent unknown – however after all the 22 stations have been compared, Proffessor Airey of Greenwich¹³ is fairly floored & will give it up in despair!

– One grand object in view was this. It was expected that the mean tide mark between Spring & neap tide would be in the same point or would coincide – whether from Bad observations or otherwise nobody knows, but the points wont agree.

Divide the distance between high & low water of Spring tide into 2 parts – call the point A – divide the distance between high & low water of Neap tide into 2 parts, call this point also A – Query, will these 2 points be on the same level – will they coinside – or will they be equidistant from the earths centre? Our observations say No – Sometimes one point would be below the other nearly 2 feet – at others nearly 2 feet above! – So far as we have gone & so far as I can discover – our experiments &c have ended in a bottle of smoke, so far as the grand point is concerned – & must remain undecided til we can find Trusty men – men who feel a personal interest in the results – and who will spare no pains to note and mark regularly the different rises & falls – it wont do to fall asleep – get tired – go on the Spree and then Fudge a Book!!! –

‘Howsomever’ from what has been done we can see the form of the wave – nearly – – We can’t discover the form or quantity of the suns wave – we only see the moons – keep in mind Sir Isaac’s calculation of the suns wave of 2 feet – Now suppose the the sun & moon to be at right angles – it is evident the suns wave of 2 feet must occupy the hollow between the moons waves and will raise that hollow (or low water) 2 feet higher than it would be – or, prevent the tide from falling so low at that place as it would otherwise do – this is theory, and our observations don’t throw any light on this point – You will see on the Tracing curious irregularities in the curve – but these are not due to the Sun or gusts of wind – or anything else except the carelessness of the observer – we might expect some irregularities from the obstruction the wave meets with – but not such as are on the tracing – The R.S.M.¹⁴ observers knew, & expected irregularities – and not to disappoint ‘Filosofers’, they made irregularities accordingly!! –

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You perceive that from low water on one side of the Earth, to low W.¹⁵ on the other side is 12.000 miles – the wave is, say 20 feet high – we have then a Base of 12.000 miles and a perpendicular of 20 feet – how can we plot that?!! – only one way, make the Base on one scale, the perpendicular on another – which will be done –

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Our observations include from 6 AM to 6 PM – taken on one gauge only – at one place – Draw parallel perpendiculars equidistant – See Tracing¹⁶ – Call each space between the lines = one hour – divide the hour into 10 parts each = 6 minutes – Look to the right of the Tracing there you have your Tide Gauge – Zero is down in the mud below low water – the perpendicular scale is any size you please – but on the trace the perpendicular scale of feet, is made to correspond with the scale of minutes – this is owing to accident – (as a piece of paper was found by the plotter, ruled, and nicely divided, he adapted the lines merely to save trouble, because if he did not use that paper, he would have to rule & divide a sheet for himself) – You commence at 6 oclock – it is evident that the space between 6 & 7 oclock represents so many miles – that is, so many degrees or miles, according to the latitude of the place, – The wave passes over you!! or suppose it so – you are supposed to stand under low W. mark, and as the Tide rolls over you – you push up your staff and take offsets to it – Just the same as if instead of walking along a crooked fence & taking offsets, you stood in one place and made the crooked fence pass by you and as it passed by, you took offsets to the crooks; if the fence was made to pass by at the rate of so many feet per second – you could take an offset at every second or every 6 seconds, and afterwards turn your seconds into feet – It is all the same in the end, whether you fly like a Thunderbolt over the Ocean at the rate of 1000 miles a second, and take offsets at every mile, down to the Sea – or whether you sit quietly down and let the Roaring ocean pass over you &c

– Each line on the Tracing is taken at the same place but each line on a different day – each colour or form of dot, represents a different tide – Now let us take one Tide, it will explain the rest, take the Black indian ink line – At 6 AM – Tide gauge read – 13 f –0 in – and there the Black line begins (on the left); we will throw away the minutes and state the hours only – –

				f	in
At	7	P.M.	Tide gauge read	15.	0 –
“–	8	“–	“– “– “–	17–	0 –
At	9	“–	“– “– “–	18–	0 –	And this 18 feet is high water – the tide begins to ebb – and
At	10-Oclock	AM–	Tide gauge =	16 –	9
“	11 –	–“	–“ =	14 –	0	and ebbs till we come to 3 oclock P.M.

when it is low water – It then begins to rise and at 6 P.M. we have high water = 12 – 2 – Thus the form and quantity of the wave at that time and at that place is plotted – Observe high waters on the left hand, how they shift their places – The high red line is farther to the left than the top of the Black line, this is caused by the wave following the moon – The Sun loses no time – 6 oclock is 6 oclock, or nearly so – the moon however loses ¾ of an hour every day – & so must her tide shift – were the series of observations carried on, you would see the tide (top of the wave) moving to the right hand ¾ of an hour every day – You will see a number of small little bends or irregularities in the curves, I can’t account for them otherwise than by supposing bad observations – were they caused by Islands, or obstructions of any kind, each wave would be similarly affected, but you see they are not all affected at the same place – You see the words – ‘Standard level of Meads Quay’ – This was only an assumed point – it does not interfere or explain anything – that standard was to be used but is to be abandoned as impracticable and (it is said) something else is to be adopted!! –

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General

It was the intention of Proffessor Airy to have all the high & low waters of the 22 stations laid down on a map of Ireland, just in the same manner we lay down our heights on a 6 inch plan – by that he could have a general view at a glance, of all the different rises – their time – and distances from each other – he could balance the quantities, or, nicely adjust them – and then strike a ‘Mean’ – he would reduce their differences to one common Datum line, and this was to have been the datum for the O.S. levels – At present this is undecided & probably will remain so for some time longer, for our observations are not very satisfactory. –

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On some parts of the Coast the rise or fall of the Tide is hardly perceptible – in the C^o. Antrim¹⁷ near Island Magee¹⁸ the fall is not more than 2 feet –

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At Courtown, C^o. Wexford¹⁹ – low water is higher than high water!! i.e. – when by the regular course of the tides round the Coast we would expect to have high water at Courtown, we have Low Water; & when we should have low W^r. we have high water, & the difference between high & low is only 4 inches!! Low water is 3 inches higher than high water –

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The difference between low water at the S end of Sligo²⁰ & Dublin Bay²¹ is only 1/100 part of a foot – so we make it –

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The tide rises at Limerick²² one foot higher than it does at the mouth of the Shannon²³ –

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Generally speaking, the tide falls or Rises 3 or 3½ inches in 10 minutes – At Limerick the tide rises 20 feet – and falls at some parts – (or during a certain portion of the fall) the tide falls one foot in 10 minutes –

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Wishing you grace, good luck & patience to read my scientific scribble – & hoping you may require some explanations! I wait in peace for your Cogitations

Yours sincly | AMcL²⁴

RI MS JT/1/TYP/11/3820-3824

RI MS JT/8/1/4a