Remarks on a Pamphlet Lately published by the Rev. Mr. Maskelyne, Under the Authority of the Board of Longitude

by John Harrison

A Publication having lately been made by the Rev. Mr. Maskelyne Astronomer Royal, under the Authority of the Board of Longitude, manifestly tending, by the Suppression of some Facts and the Misrepresentation of others, to impress the World with an unjust Opinion of my Invention, and falsely asserting that my Watch did not at certain Periods therein mentioned keep Time with sufficient Exactness to determine the Longitude within the Limits prescribed by the Act of the 12th of Queen Anne; I think it incumbent upon me to submit some Observations thereon to the impartial Publick; and the rather, because the said Pamphlet is rendered so confused by unnecessary Repetitions, and voluminous Tables, that a Man must be pretty conversant in these Matters, to trace and combine the Facts, so as to check the Conclusions, which would consequently be taken upon Trust by the generality of Readers, unless publickly contradicted. As it will be my Endeavour so far to avoid the Use of all Terms of Art as to make the Subject generally intelligible, I flatter myself I shall not be thought impertinent for giving a short Explanation (though quite unnecessary to the far greater Part of my Readers) of what the Longitude is, and what the Service required of the Watch.

The Longitude of any Place is its Distance East or West from any other given Place; and what we want is a Method of finding out at Sea, how far we are got to the Eastward or Westward of the place we sailed from. The Application of a Time-Keeper to this Discovery is founded upon the following Principles: The Earth’s Surface is divided into 360 equal Parts (by imaginary Lines drawn from North to South) which are called Degrees of Longitude; and it’s daily Revolution Eastward round it’s own Axis is performed in 24 Hours; consequently in that Period, each of those imaginary Lines or Degrees, becomes successively opposite to the Sun (which makes the Noon or precise Middle of the Day at each of those Degrees); and it must follow, that from the Time any one of those Lines passes the Sun, till the next passes, must be just four Minutes, for 24 Hours being divided by 360 will give that Quantity; so that for every Degree of Longitude we sail Westward, it will be Noon with us four Minutes the later, and for every Degree Eastward four Minutes the sooner, and so in Proportion for any greater or less Quantity. Now, the exact Time of the Day at the Place where we are, can be ascertained by well known and easy Observations of the Sun if visible for a few Minutes at any Time from his being ten Degrees high ’till within an Hour of Noon, or from an Hour after Noon ’till he is only 10 Degrees high in the Afternoon; if therefore, at any Time when such Observation is made, a Time-Keeper tells us at the same Moment what o’Clock it is at the Place we sailed from, our Longitude is clearly discovered. To do this, it is not necessary that a Watch should perform it’s Revolutions precisely in that Space of time which the Earth takes to perform her’s; it is only required that it should invariably perform it in some known Time, and then the constant Difference between the Length of the one Revolution and the other, will appear as so much daily gained or lost by the Watch, which constant Gain or Loss, is called the Rate of its going, and which being added to or deducted from the Time shewn by the Watch, will give the true Time, and consequently the Difference of Longitude.

I shall now proceed to make such Remarks as occur to me on Perusal of Mr. Maskelyne’s Pamphlet.

Mr. Maskelyne begins by telling us that the Board of Longitude, at their Meeting, April 26, 1766, came to a Resolution that my Watch should be tried at the Royal Observatory under his Inspection, and that he accordingly received it on the 5th of May, 1766. He then says, “I most Days wound up and compared the Watch with the transit Clock of the Royal Observatory myself; at other times it was performed by my Assistant Joseph Dymond, and afterwards William Baily; this was always done in the Presence of, and attested by one of the Officers of Greenwich Hospital, when he came to assist in unlocking the Box in which the Watch is kept, in order to its being wound up.”

Not one of those Attestations appears in the Book: Perhaps Mr. Maskelyne thinks his Assertion of the Fact will be sufficient for the Publick, and indeed so it might have been to me, had I not received different Information: But the Truth is, the Commissioners appointed a Set of Gentlemen to attend by Rotation the winding up of the Watch; they were to unlock the Box the Watch was in, to see it wound up and compared with the Clock, then to lock the Box again and take the Key with them, and Mr. Maskelyne was to have another Key, there being two Locks to the Box:[1] The Officers of Greenwich Hospital were appointed for this Service, some of whom from the Infirmities of Age, and Misfortunes in the Service, were scarce able to get up the Hill to the Observatory, so that when they came there, as can be proved from undoubted Eye Witnesses, they only unlocked the Box, sate down ’till Mr. Maskelyne had done what he thought proper, and then locked the Box again, and departed: and whatever Attestation they may be supposed to have made, I can prove that at several Times when Gentlemen of my Acquaintance happened to be present, the Attendance of the Officers was by no Means an effectual Check upon the Comparison of the Watch with the Clock. I would not be thought to accuse those Gentlemen of Neglect of the Duty imposed upon them; on the contrary I applaud their Diligence in being ready at all Hours of the Day to attend when Mr. Maskelyne was pleased to appoint; and therefore I will even for the present (though contrary to Fact) suppose they have been the Check proposed by the Commissioners of Longitude against any unfair Access to the Watch, still the Clock with which it was compared was left entirely in Mr. Maskelyne’s Power, and an Alteration of the one could not but produce just the same Effect as an Error of the other, nor is there even the least Pretence of a Check either on the Clock, or on its Comparison with Observations of the Sun; nay on the contrary, Mr. Maskelyne did at this Time take the Key of the Clock from Mr. Dymond in whose Custody it used to be, and kept it himself.

Mr. Maskelyne then proceeds to give us an Account of the Watch’s going from Day to Day, which in his 15th Page he concludes thus: “From the foregoing Numbers it appears, that the Watch was getting from the very first near 20 seconds per day; a circumstance which is not my business to account for; but which, as it kept near mean Time in the Voyage to Barbadoes, seems to shew that the Watch cannot be taken to pieces and put together again without altering its Rate of going considerably, contrary to Mr. Harrison’s Assertions formerly.”

When I made the Discovery, upon Oath, of the Principles and Construction of the Watch, to six Gentlemen appointed by the Board of Longitude and to Mr. Maskelyne, (who insisted on having a Right to attend, as being a Commissioner) which Discovery was finished on the 22d Day of August, 1765, as appears by the annex’d Certificate,[2] the Watch then remained in my Hands, all taken to pieces: I little imagined the Board of Longitude would take it from me, as not conceiving any Use they could make of it; and having besides received Assurances from them, that they only wanted the formal Delivery of it, in compliance with the Terms of the new Law, without meaning to deprive me of the Use of it: I therefore went on making some experiments, and alter’d the Rate of its going, thereby to determine a Fact I wanted to be satisfied about. The Watch was under this Experiment the latter End of October, 1765, when upon receiving the Certificate for the Remainder of the first Moiety of my Reward, I was ordered to deliver it to the Board. My Son, attending with it, being asked if it was then as fit as before to ascertain the Longitude, reply’d in the Affirmative; for as I have before shewn, the Rate of its going, when once ascertained, does not prevent its keeping the Longitude. He was not asked the present Rate of its going, nor could he have answer’d with precision if he had, because we had not had Notice sufficient to determine that Point; but we did, at that Time, tell several of our Friends that it went about 18 or 19 Seconds a Day, fast, and we have at several Times since (without ever dreaming that this was to become a Point of public Discussion) had Occasion to mention the same Thing to several Members of Parliament, Commissioners of Longitude and other Gentlemen, insomuch that we did not believe any body was uninformed of it, who at all attended to the Business of the Longitude.

This may serve to account for the Circumstance which Mr. Maskelyne declares, it was none of his Business to account for, why the Watch was getting near 20 Seconds per Day; but as to his Inference, I must say it betrays the most absolute Ignorance of Mechanics, and of this Machine in particular, in which it is obvious (and for this Fact I appeal to the Watchmakers who saw it taken to Pieces) that its going at the same Rate when put together again, as before, depends (if none of the Parts are alter’d) upon nothing more complicated than putting a single Screw into the same Place from whence it was taken. Indeed this Passage, and the ignorant and puerile Remarks which Mr. Maskelyne has been suffer’d to prefix to my written Description of the Watch (to the Disgrace of this Country in those foreign Translations it has already undergone) bring strongly to my Remembrance an Observation made by some of the Gentlemen present at the Discovery, “that they wonder’d at his Patience in attending so long to a Subject he seem’d so totally unacquainted with.”

Mr. Maskelyne then proceeds to tell us of a Change that happen’d in the going of the Watch, and says, “this Change began in the Beginning of August, on the few and only hot Days we had last Summer, which yet were not extreme, the Thermometer within Doors having never risen above 73°. The Rest of the Summer in general was remarkably cool and temperate.” When I took this Watch to Pieces I informed Mr. Maskelyne and the other Gentlemen, that in trying any Experiments with it, in Respect to Heat and Cold, it would be proper that it should be so fixed that, as far as could be, the Heat should have an equal Influence on all Sides of it; and it is obvious that the Thermometer ought to have been kept in the same Box with it; but as this was not done, I apprehend the Effects of Heat mention’d above do not merit much Attention; and therefore shall only observe that the Watch was placed in a Box with a Glass in the Lid and another in one Side, in the Seat of a Window level with the lowest Pane of the Window, and exposed to the South East, whilst the Thermometer, which was to ascertain the Degree of Heat the Watch was exposed to, was placed in a shady Part of the Room: Now ’tis obvious that while the Air surrounding the Thermometer might be very temperate, there might, if the Sun shone upon it, be a heat in the Box, superior to what was ever felt in the open Air in any Part of the World; and perhaps greater than any human being could subsist in, and consequently improper, or at least unnecessary for this experiment.

Mr. Maskelyne next tells us of an irregularity which he says happened in cold Weather, and says, “However, it seems in general that the Frost must have been the cause of these irregularities, as well as of the Watch’s going so much slower in the Month of January, than it had gone before.” Mr. Maskelyne ought along with this, to have published what I told him when I explained it; that the Provision against the effects of Heat and Cold was not in this Machine extended to all Degrees; that I never had tryed it so low as the freezing Point, which according to the best Informations I have been able to procure is a Degree of Cold that never did exist between the Decks of a Ship at Sea, in any Climate yet explored by Mankind.