318 



conducive to accuracy, such as is seldom attained by any other method. Those 

 who are accustomed to examine the accuracy of plotted traverses will appreciate, 

 witliout further remarks, a method by which any bearing may be checked at a 

 glance, and each traverse followed out with great rapidity and accuracy with 

 no more preliminary work than ruling one or moi"e meridian lines. 



Two forms of this instrument have hitherto been made — Howlett's 

 (mentioned above) and Metcalf 's — but neither seem to the writer to present 

 simplicity and compactness enough for general use, while, in both, the vernier 

 is set at an angle of about 30 degrees with the plane of the graduation, 

 causing great disturbance of light and shade, and, in part, sacrificing one 

 advantage of the systeui. 



In the instrument designed by the writer, [a drawing of which was 

 exhibited at the meeting] the vernier is in the same plane as the graduated 

 limb, and is distinct from the arm, woi'king on the latter in dovetailed grooves, 

 and held to the limb by a light spring ; the arm is parallel in the sides from 

 the centre joint to the extremity, excepting only the part embracing the 

 the graduated limb and vernier, which part is reduced to the smallest and 

 most compact dimensions. In deference to the custom on all protractors, the 

 centre is transparent, and means provided by which a line truly radial with 

 the centre may be marked off, but simple angles may be easier laid down, 

 precisely as bearings, using, for the sake of handiness, an ordinary short 

 parallel ruler. 



ISTo doubt very much lies in habit, but as the winter has given both 

 methods fair and extended trials, he feels justified in awarding the special 

 advantages above named to the instrument which, under the same circum- 

 stances, has enabled him to perform work the most satisfactory in all points of 



Art. LXII.- — On the Use of Salt Water in the Field Boiler. 



By James Stewart, Assoc. Inst. C.E. 



[Read before the Auckland Institute, July 11, 1870.] 



Among all the numerous inventions and improvements in steam boilers which 

 have been brought out in late years, none seem to promise better results than 

 that known as the "Field" Boiler, from the name of the inventor. This 

 boiler gives a sui-prising amount of steam, and belongs to the class having 

 water tubes. The invention, however, lies oi:ily in the tubes^ and they may be 

 applied to almost any sort of boiler now in use. The best results are likely 

 to be obtained nevertheless from boilers specially designed for " Field " tubes. 

 So far, with the use of fresh water, nothing more seemed to be desired, but it 

 very soon came to be a question, with the writer and others, whether or not 

 they could be worked with sea water. If with its use, incrustations to any 



