Completed by Flamsteed in 1683, this Mural Arc was intended as a replacement for the unusable 10-foot Mural Quadrant that had been made and supplied by Robert Hooke. Its name came about as a result of it having been made ‘too slight’ to funtion properly. It was replaced in 1689 by Flamsteed’s Mural Arc. This was of the same size and may have incorporated some of the components of the earlier instrument. No parts of either instrument survive.
Mounted on a wall in the plane of the Meridian, the ‘Slight’ Mural Arc consisted of a graduated arc of about 130º or 140º (Flamsteed states both figures) and 6 feet 9 inches radius (sometimes rounded by Flamsteed to 7 feet). It was fitted with a telescopic sight. The length of the arc meant that all the stars visible from Greenwich could be observed (something that wasn’t possible with the earlier quadrant). Used in conjunction with an accurate pendulum clock, had it functioned properly, it would have allowed Flamsteed to measure the zenith distance of a heavenly body as it crossed the Meridian as well as the sidereal time at which this happened. From this he would have been able to calculate the object’s right ascension and declination. It would also have allowed him to determine the true longitude of the equinoctial points.
Neither Flamsteed nor his contemporaries give any information about the location of the wall on which the Mural Arc was mounted. Given that it was replaced by a beefed up instrument of much the same design, it is not implausible that they were both designed to be mounted in the same place. If this is true, then instead of designing and making the instrument to be mounted on the substantial east wall of the Quadrant house (where Hooke’s Mural Quadrant had been), it would have been designed to be mounted on the opposite (west) wall. This was not only less thick; it also contained a window that would have required bricking up. The finished instrument would also have been a mirror image of what it would have been if had been designed for the east wall.
Although there is no extensive account of the instrument, Flamsteed did record how its construction came about and the limited use he was able to make of it. The following extracts in Flamsteed’s own words, have been taken from An Account of the Revd. John Flamsteed, the First Astronomer-Royal by Francis Baily.
But, finding that it was impossible to determine the true longitude of the equinoctial points, from the fixed stars, without a fixed instrument for determining their distances from the pole of the world, and that I could not probably be allowed the expenses necessary for one, by the King, at this time, I resolved to make a large mural arc at my own charge. The narrowness of my salary would not afford me to bestow much money on one: I began therefore to think how I might make it as little chargeable as could be. At last I resolved to make it near the same radius with the sextant, that it might show the meridional distances as exactly as that measures intervals in the heavens. I began it in August 1681; and the instrument itself was finished that year. But being forced to make use of an ill workman, who respected nothing but the getting of wages by his work, I found the limb faulty, and was discouraged from proceeding with it that year. The next, it lay by me also: but, the following, I brought it very near the meridian, fixed it there, and divided the limb beyond the pole. For ’tis an arc of more than 140 degrees; and I order it so on purpose that I might observe all the stars, visible in our horizon, on it with the same index. So that if (as it has happened) it should be stirred from its first position after the rectification, though it did not show the meridional heights exactly, yet it might give the precise apparent distance of any star, observed on it, from the pole. Which I find it does as well as I could expect; considering how bad a workman wrought it, and how inconvenient it was to divide it. And I am so well pleased with this part of the contrivance, that were I to make another to be placed in its room, and had all necessary expenses (which I account could not exceed £100) allowed me, I should order the limb to contain as many degrees as this does.
The fault in this instrument is, that in many places of the limb the index applies not closely to it, by reason that it warped either when first hung upon the wall, or when I forced it into the meridian. Nevertheless I conceive this fault cannot create any perceptible error in an observation; especially if due care be used in considering and copying the measure.
Finding the distances, of the fixed stars from the vertex, observed with the sextant, so uncertain and incoherent, in the year 1683 I contrived and built a mural arc of so many degrees that it might take in all the stars that passed the meridian betwixt the pole and the south intersection of the meridian and horizon; as also the pole star itself under the pole (that is, of about 130 degrees in the limb), and fixed it on a meridional wall. With this I took the distances of the sun and planets from the vertex, from that time till the autumn of the year 1686. It was built too slight, and could not be well fixed: so I durst not confide in the measures taken with it. But, however, the year following I determined the place of the sun near his mean distances from the earth, and the greatest equation of his orbit, from observations made with it, compared with others made with the sextant; confiding in the intermutual distances taken with it, though not in the meridional distances from the vertex.
My good friend Sir Jonas Moore died in August [27th] 1679: the King in February 1684. I had now no hopes of having any allowances made me for new instruments: it was well if I could keep my post, and proceed in the following reign. Some people, to make me uneasy, others out of a sincere desire to see the happy progress of my studies, not understanding amid what hard circumstances I lived, called hard upon me to print my observations. I had often answered that I had not any instrument for taking the meridional distances of the stars from the vertex, with such exactness as I could their intermutual distances from each other; that I wanted these to connect them; and that without them my labors would appear lame and imperfect. At the same time desiring them to have patience till I could afford to build such an instrument as I wanted, at my own charge; which I was resolved to do, if I could not get one allowed me by the King. In the mean time, considering that the distances I had taken, though not so exact as I desired, were much better than Tycho's; and finding the want of a better catalogue of the fixed stars than his, I set myself to calculate a small one of those most useful to me, from my own observations made with the sextant, and this slight arc ; and perfected it in the spring of the next year, 1687: having in the mean time been frequently interrupted not only by other employments, but particular affairs of my own.
Flamsteed, John. The Correspondence of John Flamsteed, the First Astronomer Royal. Vol. one: 1666-1682. Vol. two: 1682-1703. Vol. three: 1703-1719. [3 Vols.]. Compiled and ed. By Eric G. Forbes, Lesley Murdin and Frances Willmoth. Volume one: 1666-1682 . Volume two: 1682-1703. Volume three: 1703-1719.