18© Instructions for the Scientific Expedition 



It remains to say a few words of the instrumental means which 

 have been adopted for the attainment of these ends. 



The magnetic instruments belonging to each observatory and in 

 constant use, are, 1. a declination instrument; 2. a horizontal force 

 magnetometer; 3. a vertical force magnetometer. These instru- 

 ments are constructed after the plan adopted by Professor Lloyd in 

 the Magnetic Observatory of Dublin. The magnet, in the two 

 former, is a heavy bar, fifteen inches long, and upwards of a pound 

 in weight. In the declination instrument the magnet rests in the 

 magnetic meridian, being suspended by fibres of silk without torsion. 

 In the horizontal force magnetometer, the magnet is supported by 

 two parallel wires, and maintained in a position at right angles to 

 the magnetic meridian by the torsion of their upper extremities. 

 In both instruments the changes of position of the magnet are read 

 off by means of an attached collimater having a divided scale in its 

 focus. The magnetometer for the vertical force is a bar resting by 

 knife edges on agate planes, and capable of motion therefore in the 

 vertical plane only. This bar is loaded, so as to rest in the hori- 

 zontal position in the mean state of the force ; and the deviations 

 from that position are read off by micrometers near the two extre- 

 mities of the bar. 



In addition to these instruments, each observatory is furnished 

 with a dip circle, a transit with an azimuth circle, and two chro- 

 nometers. Each vessel also is supplied with a similar equipment. 

 Should therefore the ships be under the necessity of wintering in 

 the ice,-^and generally, on every occasion when the nature of the 

 service may render it necessary to pass a considerable interval of 

 time in any port or anchorage, — the magnetometers should be esta- 

 blished, and observations made with all the regularity of one of 

 the fixed observatories, and with strict attention to all the same de- 

 tails. 



The selection of proper stations for the erection of the magneto- 

 meters, and the extent of time which can be bestowed upon each, 

 must in a great measure depend on circumstances, which can only 

 be appreciated after the Expedition shall have sailed. The ob- 

 servatory at St. Helena (the officers and instruments for which will 

 be landed by Captain Ross,) will in all probability, — and that at the 

 Cape (similarly circumstanced in this respect) may possibly, — be in 

 activity by the time the ships arrive at Kerguelen's Land ; which we 

 would recommend as a very interesting station for procuring a 

 complete and as extensive a series of corresponding observations as 

 the necessity of a speedy arrival at A'^an Diemen's Land for the esta- 

 blishment of the fixed observatory at that point will allow ; taking 

 into consideration the possibility of obtaining during the interme- 

 diate voyage, a similar series, at some point of the coast discovered 

 by Kemp and Biscoe. In the ulterior prosecution of the voyage, 

 a point of especial interest for the performance of similar observa- 

 tions will be found in New Zealand, which, according to the sketch 

 of the voyage laid before us by Captain Ross, will probably be visited 



