Sergeant Steel’s first station was at Hensbarrow,[[57]] from which he was removed to South Barule, and after completing his observations there, he was stationed for a time on the wild and romantic hill of Ben Lomond. There he witnessed a phenomenon which, perhaps, had never before been seen by any one. He had frequently been above the clouds, and at Hensbarrow, of a low altitude compared with Ben Lomond, he had observed the stars a whole night when the clouds beneath him were saturating with their vapour the little village of Roach below; but on Ben Lomond he saw extensive masses of cloud settle down into a level wide-spread stratum, the upper surface of which was at least 500 feet beneath the camp. This was after sunset, on the 10th September, 1845, with a beautiful moon and a clear blue sky above, altogether presenting an impressive coup d’œil. Such was the depth and density of the mass, that it required the powerful influence of the sun’s rays for the two following days to dispel it. The whiteness of snow was grey, contrasted with the silver hoar of the heavy cloud when the sun rose on the 11th, and it offered, said Steel, in his forcible language, “a strong temptation to a lover of nature’s wildest grandeur, to treat himself to a celestial walk on its upper surface to the peak on the neighbouring hill.” Some tourists ascended the mountain on the 11th and 12th of September in the true spirit of enthusiastic enterprise, wishing to connect their names in history with this startling, yet truly magnificent phenomenon, but their amazement was indeed great, when, after penetrating the cloud, they saw above them an encampment of soldiers carrying on the official services of the station, with all the activity and fearlessness of men accustomed to such extraordinary appearances.
At Ben Heynish, in Tiree, one of the westerly isles of Scotland, the sergeant had to struggle in watching and taking a few observations between the almost incessant storms. Next he was employed in the remeasurement of the latitude of Dunnose, the southern extremity of a British arc of meridian, to verify its result as determined with Ramsden’s zenith sector in 1802, and also to test the value of Professor Airy’s sector. The observations for this purpose were carried on both at Week Down and Dunnose. “The near agreement in the results of the comparison proved very satisfactory as regards the work performed with both instruments; but to endeavour to trace the extent and amount of the disturbance that evidently affected the inclination of the plumbline at Dunnose,” the sergeant afterwards made observations both at Boniface Down and Port Valley, in the Isle of Wight, by which “the difference in the geodetic and astronomical amplitudes between Greenwich and Port Valley were found to be almost insensible, and the comparison with Boniface Down and Week Down tolerably good.” The discovery, however, of singular disagreements in the observations at “one of the stations in the Isle of Wight, which had hitherto been looked on as the southern extremity of the longest of the British arcs of meridian, and where no sensible deviation could à priori have been anticipated, led to the re-examination of the northern extremity of the same arc, situated in Balta Island, by revisiting it, and by observing also from two other stations in the Shetland Islands contiguous to it, viz., Gerth of Scaw and Saxavord.”[[58]] The disturbance alluded to—the effect of local attraction—caused the plumbline and level to be deflected or acted upon, as a loadstone would influence the needle of a mariner’s compass, and thus, when the levels indicated that the instrument was pointed at zenith, it was in fact directed to a point nearly four seconds to the north of it.
To Unst, in Shetland, the northern extremity of the arc just mentioned, sergeant Steel now repaired, and ascertained the existence of a disturbance at Saxavord, but in the contrary direction. This was fully established by taking a similar series of observations at the Gerth of Scaw, near Lambaness, and on the small uninhabited island of Balta. The relative position of these stations he fixed astronomically as to latitude, and geodetically by triangulation and levelling from the mean level of the sea, which involved observations with regard to the ebb and flow of the tides. By the series of observations so far made, it was clearly proved, that the latitude of a place could not be measured with the degree of certainty formerly supposed, and that though astronomers may profess to give seconds, tenths, and even hundredths of a second of their latitude, yet the real truth is, that the record may often be several whole seconds in error. The discovery, now confirmed by sergeant Steel’s inflexible accuracy, is likely to produce some interesting discussion in the scientific world, and has already been made the subject of an article in the ‘Philosophical Journal of Science’ for April, 1853, embraced in a review of Captain Yolland’s Sector Volume.[[59]]
After passing a station at Cowhythe Hill, in Banffshire, to verify the sector operations of 1813, and which object was satisfactorily attained, the sergeant fixed his observatory at Southampton, where, in carrying on the duty, he made various experiments to ascertain the cause of apparent errors. In taking the usual readings of the telescope micrometers, the value of the zenith point, derived from each double observation of a star, varied sensibly. To determine this more accurately, by ascertaining the true value of the divisions of the screw, and correcting the error involved in the reduction of the whole of the observation, he adopted the method of making two distinct observations of the same star without reversal of the revolving frame, in the manner described in the Sector Volume, page xxvii, and so excellent was this method considered, that the value of the screw thus obtained, was finally applied to all the observations.
In prosecuting the work, it was also evident, that the most northerly stars furnished the greatest, and the most southerly the least resulting latitudes. To arrive at the cause of this anomaly, sergeant Steel devoted much of his time to careful investigation, and his efforts and experiments were both ingenious and interesting. These embraced comparisons of the arc with Simms’ dividing engine, by which the non-existence of any sensible error in the divisions of the limb that would account for the observed errors was proved; but it was at the same time clearly ascertained, after a patient examination of the micrometer screws, the levels, the lenses, and the fullest consideration of the law of expansion by heat or contraction either by cold or pressure, that the immediate cause of the disparity arose from the compression of the divided limb by the downward pressure of the upper screw pivot, which, at each station, varied in proportion to the degree of pressure supplied. This was, ever after, a special point of attention with the sergeant, and as, from the construction of the instrument, no absolutely permanent and uniform pressure could be insured at all times, he regulated its extent as well by his judgment as his recollection.
It was a rule with him, notwithstanding the apparent errors that might be the result, to register his observations with the strictest exactness. Experience had taught him to expect them as well from local as from indefinable causes. He considered, moreover, that the more perfectly an instrument was constructed, the more honestly would it report the discrepancies of both maker and observer, and that although the conclusions would seem to be a volume of errors, more credit and merit were due to the observer for ascertaining, instead of concealing or covering his errors. Influenced by this novel consideration, he threw an amount of earnestness, care, and faithfulness into his work, that rendered his observations of the highest class for accuracy, and deserving of the fullest confidence.
At Southampton he was assisted in the sector service for nearly twelve months by corporal William Jenkins: the one observed from sunset till midnight, and the other from midnight till sunrise. His final observations were at St. Agnes in Scilly, Goonhilly Down near the Lizard Point, North Rona, and Great Stirling—the north-east peak of Scotland. By this series of observations, the arc of meridian, which before terminated at Forth Mountain in the county of Wexford, and Monach in Lewis, was extended to St. Agnes in the south, and to Rona in the north, a small, unknown, and stormy island, about 100 miles west of Orkney.
At Stirling, according to instructions, he examined the promontory to select for his observations a spot, which would be probably free from unequal attraction, and fix its position by triangulation. In this he was quite successful. The point was “so far to the east as to be out of the direct meridional line of attraction of the hills lying south of Cowhythe,” and by this series of observations it was ascertained, “that the deflection existing at Cowhythe, is not general in those latitudes, and that the discrepancy between its observed and calculated latitude, is not due to an error in the figure used in computing the geodetic result, but to local attraction affecting the astronomical latitude.”[[60]] The fact of local attraction was now fully established; but from some peculiarities of its influence in particular districts, the inference derivable from it is, notwithstanding the skilful conclusions of scientific men, that the figure of the earth is different to the commonly-received opinion of its form.
In these later services he and his party were alike exposed to dangers at sea, and to trials and privations on land; and besides encountering many perils in difficult boat service, and in landing on almost inaccessible coasts and islets, they were on several occasions nearly shipwrecked.
A small party at Rona was subjected to severe hardships. Its number consisted of corporal Michael Hayes and ten civil labourers, who embarked with sergeant Steel’s party on the 29th of August, 1850, to survey the island. On the following day, by a desperate effort, the corporal and his labourers pushed into the boat, and taking with them a little provisions scrambled amid the surf on shore; but as the weather was boisterous, and there was no harbour or anchorage in which the schooner could lie-to, she was compelled to return that evening to Stornoway with sergeant Steel and the sector party. Several days were now spent in intrepid attempts to regain the island, but such was the roughness of the sea, and such the fury of the wind, that all efforts to do so proved fruitless; thereupon, the master of the vessel considering the undertaking to be impracticable threw up his contract, and it was not until the 7th of September, when another vessel had been engaged for the service, that Rona was approached, and a landing effected. All this time, seven days and eight nights, corporal Hayes and his party were pent up in Rona upon a very scanty allowance of food, and exposed without shelter of tent or hut, or even the comfort of warm clothing, to the cold and tempestuous storms of that dreary and desolate island.