240 
MAGNETIC SURVEY IN NORTH AMERICA. 
Lefroy should be conveyed, with his assistant and instruments, in one of the regular 
canoes of the brigade from Montreal to York Factory, and that from thence he should 
be provided with a light canoe to proceed, independently of others, to Moose Fort, 
near the head of Hudson’s Bay, from whence he should return to Toronto overland 
as soon as the winter travelling should commence. By this route a circuit would be 
made completely round the supposed point of maximum of the Force, and within the 
isodynamic curve of T8. It was stated by Sir George Simpson, that the progress 
of the Brigade would admit of two hours in each day being taken for observation ; 
that four hours would be given at each of the Company’s posts, and twenty-four hours 
on the term-days of May and June; and that directions should be given accordingly 
to the conductor of the Brigade. 
Lieut. Lefroy was provided with the following instruments : — 
1. An Inclinometer of nine French inches diameter, by Gambey. This instrument 
is the property of Captain Robert Fitzroy, R.N., and is the same which was previously 
lent by him for the survey of the British Islands, and by his permission is still em- 
ployed for the public service. The needles belonging to it were the same which had 
been used in the British Survey, when they were proved to be free from index error 
at all inclinations, by the observations of Captain Johnson, R.N. and myself in the 
Regent’s Park, reported in the account of that survey. Two additional needles were 
fitted to this instrument for the purpose of determining variations of the total force 
by Dr. Lloyd’s method : they were made in 1839 by the late Mr. Robinson, and had 
been laid by in the hope that their magnetism would become steady. 
2. A Fox’s Inclinometer of seven inches diameter, with one reversing needle, and 
two needles for observing variations of the total force by Mr. Fox’s method. 
3. A portable unifilar magnetometer, for measurements of the absolute horizontal 
force, of the construction described in Lieut. Riddell’s instructions for the use of 
portable magnetical instruments. 
4. An azimuth compass of the construction proposed by the Committee for the 
improvement of ships’ compasses, and now in use in Her Majesty’s Navy ; the bowl 
was reduced in height to render it more portable. 
5. A portable declinometer. 
6. A portable bifilar magnetometer ; and 
7. A portable induction inclinometer. These three instruments were designed for 
observations on term-days, and for the observations of diurnal variation and of dis- 
turbances in case of detention at any of the Company’s posts. 
Lieut. Lefroy was also furnished with a Dollond’s repeating and reflecting circle 
and mercurial horizon, for determining geographical positions ; with meteorological 
instruments, including an apparatus for measuring heights by the boiling-point of 
water ; and with two pocket chronometers, which were kindly lent for the occasion 
by the Ilydrographer of the Admiralty. 
Lieut. Lefroy met Sir George Simpson at Montreal towards the end of April 
1843, and embarked with Bombardier Henry and the instruments on board one of 
