84 Mr. R. W. Fox's Notice of an Instrument for ascertaining 



The deflector is furnished with verniers at its extremities. 

 To ascertain the magnetic intensity, let the deflector (with 

 the tubes closed, for example,) have its poles turned round so 

 as to coincide as nearly as possible with the dip of the needle 

 at the place of observation; the latter will be deflected either 

 to the right or left (the amount of this deflection should not, 

 I think, be much less than 60° or 70°); produce vibration 

 and then note the place of the needle, observing it at both 

 ends. Draw out one of the small tubes for a moment, till 

 the needle swings back beyond the line of the dip; then quickly 

 return the tube to its former position, and the needle will 

 be deflected in the opposite direction : half the sum of these 

 deflections will represent the intensity of the terrestrial mag- 

 netism at the place of observation. This should be repeated 

 with the face of the box turned the opposite way, the deflec- 

 tor being made to coincide with the dip as before. The small 

 tubes and the deflector being turned round 180°, the obser- 

 vations should be again repeated in like manner, and will 

 tend to correct the previous results. In this case it is evident 

 that the poles of the deflecting magnets are opposed to the 

 direction of the terrestrial magnetism, whereas before they 

 coincided with it, and its influence on the force of the magnets 

 becomes hereby compensated. The deflectors may be em- 

 ployed in attracting instead of repelling the needle, by being 

 placed at right angles or at any other given angle to the 

 dip; but there are many reasons, I think, for preferring the 

 former. The deflector may also be used with great advan- 

 tage in correcting the observed dip, inasmuch as the force 

 of the earth's magnetism on the needle increases in the ratio 

 of the sine of the angle of deflection. This it may effect 

 in different ways; but a small deflection appears to me to 

 give the most satisfactory and decisive results ; and the force 

 of the deflector may in this case be modified by drawing out 

 both tubes, more or less, and adjusting it at any given angle 

 on each side of the dip. The effect of temperature on the 

 deflector and dipping-needle may be easily ascertained by 

 covering the instrument with a double metallic case, contain- 

 ing water at any required temperature, an opening being left 

 in it for the purpose of observation, which should be covered 

 with glass. It will, however, be sufficient for the maker, to 

 ascertain what corrections should be made, as it respects each 

 instrument, on this account. I have employed such an ap- 

 paratus, and found that a needle repelled by a deflector at 

 an angle of 61° 30' from it when the temperature was 52°, 

 stood at 61° 15' when the temperature was raised to 95°, 

 and returned to 61° 30' on cooling; so that the amount of 



