390 



Grubb — Some Netv Forms of Oeodetical Instruments. 



can satisfy himself that he is holding the instrument level by watching the 

 position of the bubble, at the same time that he ascertains the coincidence of the 

 fiducial line and the reading of the staff, while the gradient shown in the upper 

 part of the field is read off on the circular arc. 



If desired for more accurate work, such instruments can be mounted upon 

 parallel plates and tripod stand, but it is in rapid work of an appi-oximate 

 character that it bears such a favourable comparison with the existing forms. 



Prismatic Compass. — A section of 

 the Prismatic Compass, with the new 

 sight attached, is shown in fig. 4, and 

 the instrument is shown in use in PI. 

 XXXIV., fig. 9. In this case the only 

 alteration, so far as the compass box is 

 concerned, is that it is desirable to have 

 the card printed upon a transparent sub- 

 stance such as celluloid, and with trans- 

 parent lines and figures upon a black ground. This enables the full light from the 

 sky to be utilized, and the divisions of the card are then seen projected perfectly 

 sharply on the object, whose magnetic bearing it is desired to ascertain. 



There is, however, in this compass a peculiar feature of special importance. 

 It will be observed that there is no fiducial line whatever, nor is this necessary, 

 because the instrument is so constructed that the degree divisions upon the card 

 are seen j^rojected upon the object, representing actual degrees upon the horizon, 

 as seen from the observer's station. Therefore when the card is stationary the 

 bearing of every object in the field corresponds to the particular division on the 

 card which superposes on it, whether that object be in the centre or at the sides 

 of the field. This feature is of very great importance as it simplifies the observa- 

 tions considerably. In the ordinary way it is necessary to make two coincidences, 

 first, between the very indistinctly seen thread and the object, which is itself 

 also indistinct, and secondly, between the indistinctly seen thread and the compass 

 card ; whereas in this new form there is only one coincidence required to be made, 

 that between the object itself and the division on the card, both of which can be 

 seen absolutely sharp at the same time. 



A Clinometer on this principle is of essentially the same construction as 

 the Prismatic Compass, except that the compass card and magnetic needle are 

 replaced by a divided circle or arc weighted at one point, and that the instrument 

 is held with the box in a vertical instead of a horizontal plane. This constitutes 

 a clinometer of very convenient form. 



