TRANSACTIONS Of SECTION fi. 767 



fitted witli a small circular arc and can be placed iu a line with the rulef, when 

 the instrument can be used as an ordinary telescopic plane-table alidude. Larger 

 sizes of the distance -finder, with longer rods, are made fitted to a stand as a 

 separate instrument, and the instrument can be combined with a theodolite for 

 traversing. 



2. The Astronomical Compass and Time Indicator. — -This is a simple and in-, 

 expensive little instrument for quickly finding the north and south line and the 

 true bearing of any object, as well as the local mean time by the sun or stars with 

 sufficient accuracy for all ordinary purposes. It has been designed by me to meet 

 the requirements of travellers and for night-marching when the ordinary magnetic 

 compass cannot be relied on. 



The instrument consists of two small discs, upon each of which is a projection 

 of a hemisphere drawn on the plane of the meridian. One of the discs shows, in 

 Mack, parallels of declination of the principal stars and of the sun at different 

 dates, with hour circles subdivided into five minutes, while the other, the upper 

 one, is transparent and shows, in red, curves of altitude and azimuth, The two 

 discs are centered on a pivot, and tbe transparent one is weighted at the lowef 

 part to keep it vertical. Through the central pivot passes a pin, carrying a sight- 

 rule or alidade on the front of the transparent disc and fixed to the handle at the 

 back. 



"When the instrument is used as a compass the N. or S. pole of the decli- 

 nation circle is first set to the red number on the transparent disc equal to the 

 latitude of the place; then, allowing the discs to swing freely, clamped together, 

 a rough altitude of the sun is taken through the sight-rule. The altitude circle 

 indicated by the pointer at one end of the sight-rule is now followed across the 

 disc until it intersects the curve of declination of the sun or star observed, seen 

 through the transparent disc, and the red longitudinal curve passing through the 

 point of intersection gives the true bearing or azimuth of the scar or sun at the 

 time. Then, considering the upper disc as a compass card with the zenith as the 

 true N. point, the sight-rule is set to this bearing, counting the degrees round 

 the circumference of the disc. Now when the discs are held horizontal with the 

 sight-rule aligned on the sun or star observed, the true north will be indicated by 

 the large red arrow on the transparent disc. There is an additional movable 

 pointer which can he previously set to any bearing upon which it is desired to 

 march. The lines are all bold, so that they can be read ofl' easily at night. The 

 whole operation is extremely rapid, so that bearings at intervals of a quarter of an 

 hour or so can be taken without delay. 



The same observation that gives the true bearing also furnishes the time, since 

 the hours and minutes read oft' the black figures of the declination disc at the 

 point of intersection indicate the hour angle of the sun or star. This hour angle, 

 of course, in the case of the sun gives the local apparent time, which can be 

 converted into mean time by applying the equation of time taken from the table 

 at the back, where a diagram is also given from Avhich the approximate local mean 

 time can be quickly found without any computation from the hour angle taken 

 off as in the case of the sun. 



3. The third instrument to which attention was called is a new form of reflecting 

 artificial horizon. It is intended for land or sea use, and it is hoped will be of service 

 in Polar exploration on moving ice. The special advantages claimed for it 

 over other attempts at the same kind of instrument are (1) that by the system 

 of taking observations errors are counterbalanced and a good mean obtained ; 

 (2) that it can bo readily fitted on in front of a sextant when required for use, 

 and afterwards taken off; (3) that no arrangement is required for illuminating 

 wires for star work. Meridian altitude latitudes on land have been frequently 

 taken with this artificial horizon to within 8" or 10" of the truth, and on sea 

 the results should be near enough to he quite useful. 



This artificial horizon is very simple in construction, consisting merely of a 

 reflecting mirror kept horizontal by a short weighted pendulum, of which the 

 weight is slightly below the centre of gravity. The mirror is adjusted to move 

 accurately iu the ^laue of the index and horizon glasses of the sextant by means 



