110 



♦ KNOWLEDGE ♦ 



[March 1, 1887. 



short dark vertical lines del are the 5' divisions on the limb 

 of the instrument itself. The comb c does not move with 

 the spider-lines to be immediately referred to. The object- 

 glass of the microscope is screwed up or down until five 

 revolutions of the milled head carry the crossed spider-lines 

 s L exactly from one division to the next. It must be, of 

 course, thoroughly understood that tlie microscope itself is 

 a fixture, and that the telescope and circle moving together 

 carry the divisions on the latter across the field of the 

 microscope ; and if we suppose the middle notch of the 

 comb c and the intersection of the spider-threads to be in 

 a line, and the zero of the circle on the milled head to be 

 opposite to its index (not shown in the figure), then will 

 this intersection of the crossed spider-lines become a fixed 

 point of reference, and the divisions which pass it as 

 the telescope and circle rotate together will show the 

 angle through which the telescope has moved. Let us 

 now suppose, though, that instead of such rotation bringing 



Fig. o. 



the cross accurately on to a division, it falls between two 

 of them. In this case we turn the milled head until the 

 intersection of the lines coincides with the graduation 

 hcyond wliich the motion of the telescope and circle has 

 carried it, and read oflf the revolutions and parts of a 

 revolution made by the milled head. Let us take an 

 example. AVe have said that each gi-aduation on the limb 

 of the instrument represents 5' ; and that exactly five 

 revolutions of the milled head carry the ci'oss-wires from one 

 of such graduations to the next. Moreover it will be 

 remembered that the milled head itself is divided into 

 60 parts. Now let us imagine that the graduation reads 

 274° 15', and a certain quantity equal in terms of the 

 micrometer to three whole revolutions (measured on the 

 comb c) and 41 '7 divisions of the milled he.ad. This 

 quantity, bj' which the cross-wires have to be moved back 

 to coincide with the 15' division obviously is equal to 

 3' and '^g^th of a minute, or 41 "-7. Adding this, 

 then, to the whole reading we say 274° 15' -f 3' 41 "'7 

 = 274° 18' 41"'7. If the reader has thoroughly grasped 

 the principle of this instrument — and we would urge 

 him to read and re-read our description until he does 

 — he will see how, by making the thread of the screw 

 very fine and the micrometer head very large, and, more- 

 over, graduating the latter sufliciently minutely, there is 

 scarcely any limit to our power of subdividing space. 

 Suppose, for example, that the frame in fig. 5 were moved 

 by a screw sc with 100 threads to the inch, and that the 

 head M h (instead of having 60 divisions) were divided into 

 100 parts. Then, quite obviously, the turning of the screw 

 through one of these parts would move the frame 100th of 



100th— in short, 



,,th of an inch. An instructive illus- 



tration of the use of such a micrometer is found in the 

 operations conducted between 1843 and 1854 for the con- 

 struction of a standard yard-measure to replace that destroyed 

 in the fire which consumed the Houses of Parliament in 

 1834. It is to the indefatigable labours of the late Messrs. 

 Baily and Sheepshanks that we are mainly indebted for the 

 possession of our present national standard, though as 



Mr. Baily unfortunately died a year after the commence- 

 ment of the work, the overwhelming proportion of 

 it devolved on Mr. Sheepshanks. The standard which 

 had been destroyed was made by Bird, the optician, 

 in 1760, from one, also of his construction, made in 

 1 758 ; the last in turn having the Exchequer yard of 

 Queen Elizabeth, constructed in 1588, and the yard of the 

 Eoyal Society, dating from 1742, as its prototype. We 

 have spoken just now of "the standard which had been 

 destroyed " ; but, as a matter of foct, both of Bird's yards — 

 that of 1758 as well as that of 1760 — were destroyed or 

 irretrievably damaged in the fire in 1834. But other means 

 of reference happily survived. Imprimk, there was General 

 Roy's 42-inch measure, constructed by Bird, and first used 

 for the measurement of that base on Hounslow Heath which 

 was the very beginning and foundation of the Ordnance 

 Survey. The first 36 inches of this were alone employed in 

 subsequent comparisons, or rather two copies of that part of 

 it. Then there was the yard constructed b^' Troughton 

 under the direction of Sir George Shuckburgh, and a yard 

 made by Capt. Kater in 1831. Formerly comparisons would 

 have been made by meansof beam-compasses, which, as most 

 people are aware, consist of a rigid and aljsolutely straight bar 

 of hard wood or metal, to one extremity of which is fixed a 

 fine point at right angles to its length ; while along the bar 

 slides a box carrying a similar point, which can be moved 

 by a screw, a vernier attached to the box showing the 

 precise distance that the fixed and movable points ai'e apart. 

 But this, after all, was only a rough and ready method of 

 taking off distances, and one open to the most serious ob- 

 jection that the mere in.sertion of the points of the compasses 

 into the lines or holes defining the ends of the measure 

 sufliced seriously to impair the graduation, to make the 

 lines when magnified into ruts, and the points into irre- 

 gularly-shaped pits, and so to gravely impair the value of the 

 original as a standard. Instead, then, of the material beam 

 compass, with its mischief-working points, the Commissioner 

 employed what we may call an optical one in the .shape of a 

 bar furnished with two micrometer micioscopes rigidly fixed 

 36 inches apart, beneath which the original bar was carefully 

 supported, and bisections of the dots on it made hy a con- 

 ti'ivance analogous to that described above. By the sub- 

 sequent substitution of the bar which had been copied from 

 this original, the necessarily minute marks were brought 

 under the microscope, and thus a series of comparison 

 measures instituted. More than two hundred thousand (I) 

 micrometer readings were taken from first to last, the larger 

 part of them by Mr. Sheepshanks in the cellars of the old 

 rooms of the Royal Astronomical Society in Somerset 

 House, where the comparing micrometrical apparatus 

 was fixed by Messrs. Troughton and Simms. We may 

 add that one division of the micrometer head of this 

 particular apparatus represented a movement of TTiy^jnyth 

 of an inch. A similar application of what we have 

 designated an optical beam compass is employed, in 

 geodesical operations, in the measurement of the fundamental 

 base lines. Such bases are, of course, measured ligidly in 

 straight lines by rods most ingeniously devised to preserve 

 an invariable length — a device which it may be worth while 

 to digress for a short time to describe. There is a fact 

 familiar to every one which is vulgarly and forcibly (if not 

 quite philosophically) expressed by saying that " bodies 

 expand liy heat and contract by cold." In the case of the 

 standard measures of which we have been speaking, 

 it is definitely understood that the distance between the 

 lines or dots on them is precisely 1 imperial yard at a tem- 

 lierature of 62°. If the thermometer rises above this, the 

 interval exceeds 36 imperial inches. If it falls below that 

 temperature, then is such interval less than a yard by an 



