112 



NATURE 



[July 



?, 1909 



equal to the zenith distance of the star expressed in 

 minutes. 



A search at the Patent Office library showed that a 

 large number of inventors had for nautical purposes, rather 

 than for use in balloons, imagined instruments which, for 

 various reasons, would be impracticable. In some an 

 attempt has been made to combine a sextant and a pen- 

 dulum, but even if the observer were not expected to watch 

 the star and the pendulum at the same time, the pendulum 

 was made so short and of such quick period that the 

 inevitable trembling- of the hand would give rise to angular 

 relative movement of the pendulum represented by several 

 diameters of the sun. The beauty of the sextant is the 

 property it possesses of gluing- the two objects, e.g. 

 the sun and horizon or moon and star, which are being 

 observed together, so that with all the spasmodic move- 

 ments which the magnification of the telescope and the 

 unsteadiness of the hand make inevitable, the eye, never- 

 theless, can follow them and see if there is continuous 

 close contact or not, whereas if the apparent position of 

 one of the objects only depended upon the steadiness of 

 the hand, no observation worthy of the name would be 

 possible. It is therefore essential, if any approach to 

 accuracy is required, that the star or sun should be seen 

 in the same field with, and glued to, the mark, what- 

 ever form that may take, which determines the 

 altitude, and also that the angular variation in 

 the position of this mark should hardly be afifected 

 by the trembling of the hand. I tried at the 

 time to interest one or two instrument makers, 

 but unsuccessfully ; now, however, that the subject 

 is attracting attention in Germany, as shown by 

 Dr. Lockyer's (vol. Ixxx., p. 29) article in a recent 

 number of Nature, perhaps my design may be worth bring- 

 ing forward. I would only remark that an instrument of 

 the kind would be useful on board ship when the sun or 

 stars may be visible while the sea horizon is obscured, 

 provided only that, as is usual in fog, the ship is not 

 rolling seriously. These worse conditions can only be met 

 by the more complicated gyroscopic horizon perfected by 

 Admiral Fleuriais. 



The instrument depends essentially upon the use of a 

 vertical collimator suspended on gimbals, and top-weighted 

 like a metronome, so as to have a period of swing either 

 way of as much as one second. The collimator has at its 

 focus a scale of, say, tenths of a degree in transparent 

 divisions upon an opaque ground, and above its lens a 

 clear or half-silvered glass mirror set at 45° with the 

 axis of the collimator. The collimator is suspended in a 

 tube, which is the handle of the instrument, and which 

 carries also the parts of a small sextant. 



Figs. I and 2 are vertical sections through the axis of 

 the instrument, the latter partlv in elevation, a is a box 

 frame to which are attached the tubular handle h the 

 telescope c, and other sextant parts. The telescope is 

 carried by means of a slide d and pin e, so that it may 

 be moved sideways or be hinged downwards when not in 

 use. Inside the handle is mounted a gimbal ring /, on 

 which the collimator g is supported on knife-edges' h ■ 

 I IS the scale already described; k is the unsilvered mirror 

 attached to the collimator, by means of which the scale 

 «, illuminated by the mirror 7, may be seen in the tele- 

 scope; ; and n are the horizon and index glasses re- 

 spectively of the sextant, but made as prisms for con- 

 venience, though, of course, the usual mirrors might be 

 used; r is the top weight of the collimator; and t a 

 correcting weight running on a screw to bring the zero 

 of the scale 1 apparently on to the true horizon. A 

 conical damper «, lined with velvet, is made to slide 

 within the handle, being pressed upwards bv a spring v 

 so as to steady or even to lift the collimator off its -y's and 

 against the pins i, and capable of being moved down- 

 wards by the thumb-lever .r and fork y. An exterior 

 sleeve 5 carries a cap 8, which serves as a protector to 

 the translucent window at the base of the handle, and 

 as a holder also for the illuminating mirror 7 • ■? is a 

 quadrant carrying three dark or tinted glasses. ' 



When the telescope is directly opposite the mirror Ti 



and the reflectors /, m of the sextant, the star will be 



seen by double reflection projected upon the scale, of which 



one half ,s marked + and the other -. The arm of the 



NO. 2073, VOL. 81] 



sextant being therefore set to any position to bring the 

 star on to the scale, a scries of scale readings may then 

 be rnade, which, added to or subtracted from the vernier 

 reading, give the series of altitudes. If the telescope is 

 slid sideways so that half its field is to the right of the 

 mirror k, it may be made to look into the object-glass 

 end of a surveyor's level or even at the sea horizon with 

 a known dip, and the zero of the scale tested and so 

 adjusted by means of the moving weight z. At any time 

 when a sea or artificial horizon is available, observations 

 may be made as with an ordinary sextant with the 

 telescope laterally displaced, and by this means also the 

 index-glass may be adjusted. 



I have experimented with a collimator and telescope 

 mounted as described, and found that, without the top 



Fig. 



Fig. 



weight, the angular movement due to unsteadiness of the 

 hand is far too great for accurate observation, but that 

 when the period is increased to about one second by top- 

 weighting, the angular movement is so far reduced that, 

 when sitting at a table and holding the instrument in 

 the hand, an accuracy of i' is possible. Of course, with 

 the trembling of the hand the collimator turns about its 

 centre of oscillation, and so with the period named a 

 sudden movement of i/ioo inch will correspond to i' 

 about, while if the period is two seconds the angular 

 accuracy will be four times as great. 



If used on a ship with any appreciable rolling it would 

 be best to get down to the neutral axis, and observe zenith 

 stars through a hatchway, so as to avoid the horizontal 

 acceleration which is so pronounced on the bridge, for, of 

 course, the collimator will hang, not in the true vertical, 



but at an angle equal to tan 



_i horizontal acceleration 



If this is small the star may be observed to move a 

 corresponding degree upon the scale in time with the 



