TRANSACTIONS OF THE SliCTIONS. 45 



The advantages claimed for this little invention are : — 



1. Simplicity in the mode of observing-. — The author maintains that observations 

 can be more perfectly made with a sextant by allowing the objects to come into con- 

 tact, and noting the moment of contact, than by bringing them into contact and 

 noting that time ; thus the observations of the traveller inexperienced in the use 

 of the instrument will prove of more value by this mode of observing than by that 

 usually followed. 



2. In star observations. — Every observer knows full well the difficulty attending 

 taking star observations, the trouble in keeping the lamp trimmed, then that of bring- 

 ing the focus of the light on to the vernier in reading off, and the delay consequent. 

 There is also a phj'sical difficulty ; viz., in obsen'iug, the pupil of the eye has to be 

 dilated to take in the greatest possible quantity of light, and suddenly contracted to 

 exclude it in reading off, to be as suddenly changed again. These difficulties, the 

 author believes, are avoided by this simple adaptation. If circummeridian alti- 

 tudes are being_ observed, all the altitudes before and after crossing the meridian 

 are equal ; and if it be necessary to record the meridian altitude itself (which may 

 occur between the clicks), it can be done by the indicator before the arm ; but the 

 meridian altitude is not absolutely necessary. 



3. Two sets of star observations can be made by the same sextant without reading 

 off, provided their altitudes are not the same. — Having taken the iirst set Csay the 

 one with the lowest altitude), the indicator behind the arm is left to record it, and 

 the indicator before the arm will record the other. 



4. In equal altitudes of the sun, before and after noon, for time. — After taking 

 those in the forenoon, the sextant may be left until the last observation taken comes 

 on in P.M., and the altitudes respectively worked back to the first of the forenoon. 



5. In lunar observations. — Every observer of limar distances on board ship knows 

 the difficulty attending taking these observations. When there is much movement 

 in the vessel it takes some time to get the sextant on ; but when once it is got on 

 the proper angle, he can keep the objects in contact. By means of the micrometer 

 he is not necessitated to remove the sextant from the eye, and can go on taking his 

 distances ad libitum. 



6. In thick or cloudy, or even rainy weather, when a heavenly body can only be 

 seen for a short time, the observer is not dependent on one observation, but can 

 take a set in less time than he could one or two by the ordinary process. 



7. The check on the time-taker. — A good observer has a difficulty in checking 

 his time-taker. The process to detect error is rather long and complicated ; but the 

 measurements of arc being equal by the micrometer, an error in time is at once 

 detected. 



8. In nautical surveying. — The indicator attached to the ordinary sounding quin- 

 tant will prove useful by enabling the two angles, to tix a position, being taken 

 without removing the sextant from the eye, and thus avoiding the necessity of 

 having two obseners (often necessary), or the use of a double sextant. 



On an Instrument for the Composition of two Harmonic Curves. 

 By A. E. DoNKiN, M.A., FeJhiv of Exeter College, Oxford. 



Sincea simple harmonic curve may be regarded as the cm-ve of pressure on the 

 tympanic membrane when the ear "is under the influence of a simple tone, a 

 curve compounded in the ordinary way of two such harmonic curves will be the 

 curve of pressure for the consonance of the two tones which they severally 

 represent. 



Hence a machine which has for its object the composition of two harmonic 

 "curves, possesses the means for rendering distinctly visible to the eye the effect on the 

 ear of the consonance of auA' two simple tones. 



If a pencil-point performs rectilinear harmonic vibrations upon a sheet of paper 

 moving uniformly at right angles to the direction of these vibrations, it describes a 

 simple harmonic curve. If there be now given to the paper, in addition to its con- 

 tinuous transverse motion, a \ibratory motion similar and parallel to that which 

 the pencil has, a complicated curve will be the result, whose form will depend on 



