138 Rev. W. Hodgson on New Applications of Prisms. 



taken in the direction A C as well as in the direction C A 

 (fig. 1), and as each face of the prism may be used in succes- 

 sion, there are altogether six observations so arranged that 

 their errors will correct each other. Figs. 2 and 3 are 

 rough sketches copied from an instrument fitted for observ- 

 ing any horizontal angles and some altitudes. The table for 

 supporting the prism and its frame consists of a stout square 

 brass plate, adjusted for parallelism to the horizon by the 

 two spirit levels A A, and by the three screws B B, of 

 which two only are shewn in the figures. The upper surface 

 of the table of the instrument is graduated ; and the frame 

 which contains the prism is clamped by a screw to the 

 movable circle which carries the verniers. "With this 

 arrangement the horizontal angle between two objects is 

 readily found. It is not material that the objects should be 

 nearly at the same height above the horizon ; for the only 

 effect of viewing an object obliquely, instead of directly 

 through the prism, is a very slight displacement in a vertical 

 plane, which does not interfere with the horizontal angle. 



In fig. 3 the back of the same instrument is shewn in a 

 state adapted for finding the time, &c., by the method of 

 equal altitudes. On the face of the movable circle already 

 referred to, there are two small holes to receive the points 

 G G, by which the axis of the prism is kept parallel to the 

 plane of the table. The screw C C which passes through the 

 nut D, gives the power of raising or depressing that face of 

 the prism through which the observation is being taken. The 

 nut D is movable upon its axis, and the point F of the 

 screw C C, may be inserted successively into the several holes 

 which are prepared for it. By this means any required num- 

 ber of altitudes may be taken, and as these can be all repeated 

 in the reverse order, these will be the pairs of observations 

 which are necessary. 



If the actual altitude in any case is desired instead of the 

 mere fact of there being pairs of equal altitudes, this may be 

 obtained by a slight modification of the instrument. Let the 

 nut D be rigidly fixed so that the axis of the screw C C shall 

 be perpendicular to the face of the prism used in taking the 

 observation. Then the tangent of the altitude to the constant 



