28 PROFESSOR FORBES'S EXPERIMENTS ON 



to that circumstance alone that I impute the consistency of the results obtained, 

 and what I am inclined to consider the first determination worthy of confidence 

 of the Decrease of the Horizontal Intensity with height above the level of the sea. 

 If this effect is caused or modified by a variation of the dip, that investigation 

 remains open to any future observer who is prepared to undertake so very difficult 

 an inquiry. For the present we must be content to know the fact, that the hori- 

 zontal part of the intensity diminishes as we ascend. 



55. These observations, of course, have reference only to the particular 

 methods of obtaining the dip and horizontal intensity which I have exclusively 

 employed ; namely, a statical method for the dip, and a dynamical one for the 

 intensity. Professor LLOYD'S elegant statical method of determining both elements 

 at once, must of course be judged of on its own merits, and the same remark is 

 applicable to the excellent results which Mr Fox has obtained with his Deflector. 



56. The Council of the Royal Society of Edinburgh having agreed to provide 

 a portable Dipping Needle to accompany their HANSTEEN Apparatus, one with a 

 circle of six inches clear diameter was constructed under my directions by Mr 

 ROBINSON of London. That size was selected in order that its bulk might not 

 render it useless to the mountain traveller, and because I had been led, from pre- 

 vious experiment, to suspect that increase of size beyond a certain moderate 

 limit is of little or no advantage in making dip observations. Increase of weight 

 produces increased friction both directly, and because the steel axis requires in- 

 creased strength, and therefore a larger diameter ; and this probably out of pro- 

 portion to the increased directive power of the needle's magnetism. My instruc- 

 tions to Mr ROBINSON were to make the needles with the most delicate axis that 

 he could get a chronometer-maker to execute, indicating at the same time a very 

 obvious construction by which (as in all modern needles the agate bearings are 

 very narrow) the general strength of the axis may be made such as to avoid any 

 chance of flexure by the weight of the needle, or any trifling accident. The work- 

 ing of the instrument more than satisfied my expectations, and I am inclined to 

 think, judging from the detailed reports of observations made with dipping needles 

 of larger sizes by the best makers, that the Royal Society's six-inch needle (which 

 is arranged so as to pack into a mahogany-box only 10 * 8 * 2-? inches external 

 dimensions, and weighing 9 lb.), is capable of doing very nearly, if not quite as 

 good, work as any hitherto made of larger dimensions. 



57. I state this as my present belief, but I will enable the reader to judge. 

 At the same time I speak of the needle when in perfect adjustment, recently from 

 the maker's hands, for the effects of incessant jolting in long land journeys is very- 

 marked in deteriorating its performance. The best of the two needles which Mr 

 ROBINSON has furnished (and let it be stated, to the credit of that excellent artist, 

 that he is the first in this country who has vied with the workmanship of GAMBEY 

 in the construction of this most troublesome instrument), gives results which I 



