XXVIII INTRODUCTION. 



and long, with, front and back of parallel glass. The box is fastened to the oblong plate by 

 keys and pins which pass through it, and is removable at pleasure. The columns within the 

 box sustain two horizotal bars 0.6 inch apart, fitted with agate planes. Between them there is a 

 loose frame, with a pair of Y's in the middle, a hinge at one end and projecting pin at the other, 

 operated by a milled head outside of the box, and for more convenient manipulation of the 

 needle the front of the box is hinged. There is a spirit-level on the plate between the columns, 

 and a clamp-screw, by which it may be secured to the horizontal circle. 



The other pair of columns mentioned four inches high and five and a half inches apart 

 sustain a vertical circle of six inches diameter. This is divided upwards and downwards, from 

 to 180, on a band of silver, to half degrees, which are subdivisible to single minutes of arc by 

 two diametrically opposite verniers. These verniers are at the extremities of movable radius bars, 

 carrying reading microscopes three and a half inches apart, each microscope having three paral- 

 lel lines engraved across a disc of thin glass in the common focus of their object and eye lenses. 

 A third radius bar at right-angles to the others is furnished with a clamp and tangent screw. 

 When the verniers read the central lines of the microscopes are in a horizontal plane above 

 the surfaces of the agates equal to half the diameter of the axles of the needles, and equi-distant 

 from the Y's, to effect which adjustment both the microscopes and agate planes are furnished 

 with suitable capstan-head screws. 



The instrument has two needles, marked A 1 and A 2, respectively. Each is 3.5 inches in 

 length, 0.1*7 of an inch broad in the centre, 0.033 of an inch thick. Their axles are 0.88 of an 

 inch long, the extremities that rest on the planes being 0.025 of an inch in diameter. When 

 in place, the ends of the needle are at distinct vision of the microscopes. 



An extra bar accompanies the inclinometer, to be used, in accordance with a suggestion of 

 Dr. Lloyd, for measuring the absolute total force in localities where the inclination approaches 

 90, and where the usual method of ascertaining this element from its horizontal component 

 ceases to be satisfactory. The extra bar is to be attached to a prolongation of the radius bar 

 which is fitted with clamp and tangent screws, and is so constructed that the graduated tube in 

 which it terminates shall be in the same vertical plane as the inclination needle when in place. 

 A counterpoise, to be attached above the clamp and tangent screw, in this use of the instrument, 

 preserves it in equilibrium. These portions of the inclinometer were never used by us. 



A pair of nine-inch bar magnets and a portable stand also came with it. The former com- 

 menced to lose their magnetism immediately ; and as there was no facility for properly re- 

 magnetising them in Chile, a new pair was ordered from Mr. Thomas Jones, Charing-Cross, 

 London. A similar difficulty having been experienced on more than one previous occasion, in- 

 structions were sent to insure, if possible, bars that would retain their strength. But the 

 second pair proved no better than the first; and, though every care of them was taken, within 

 less than three months neither single magnet would lift one of the needles. This made the 

 observations both tedious and unsatisfactory, because, whilst the needles oscillated longer for 

 want of saturation, the individual readings were invariably more discordant. Another source 

 of annoyance was the want of a tangent screw to the horizontal circle. 



MODE OF OBSERVATION. 



Observations of the inclination were made as follows : After the instrument had been levelled 

 on its tripod-stand, the needle designed for experiment was placed on the agate supports, with 

 its marked side next to the microscopes, and the vertical radius bar was turned until the upper 

 microscope read 90. The vertical circle was then turned in azimuth,, with the microscopes to 

 the south, until the upper end of the needle was bisected by the middle line of the correspond- 

 ing microscope. If it continued to be so bisected after raising the Y's, and lowering the needle 

 again, the reading of the horizontal circle was noted. The vernier of the lower microscope was 

 next set at 90, and the reading of the horizontal circle recorded when the south pole had been 



