a Theory of the Earth. 219 
10, A compals, furnifhed with a crofs ftaff, to find the di- 
rection of mountains, chains, vallies, and ftrata. 
tr. Portable barometers with two mercurial thermome- 
ters; one affixed to a barometer to cftimate the temperature 
of the mercury in the latter, and the other with a bare bulb 
for meafuring the temperature of the air. Thofe who ftudy 
meteorology, as well as geology, ought to be furnifhed alio 
with an bygrometer and an electrometer. 
12. For afcertaining the temperature of the fea, at great 
depths, it will be neceflary to have a thermometer conftructed 
like that deferibed‘in my Travels through the Alps*: for 
Jakes, an apparatus like that pointed out ia the note of Sec- 
tion 1399, will be fufficient. am, 
13. Thofe who underttand a little geometry, ought to pro- 
vide themfelves with a fextant, having an artificial horizon, 
and alfo a chain, in order that they may be able to meafure 
a bafe, and thus take the altitude of an inacceflible peak, the 
breadth of a river, &c. &c. With this fextant they may 
alfo find the latitudes. In regard to the longitudes, they re- 
quire, befides in{truments, an expertnefs in this kind of ob- 
fervation, which cannot be attained but by mariners or pro- 
feffed aftronomers. 
14. It will be neceffary alfo to have within reach tools for 
tepairing an inftrument in cafe it fhould happen to be de- 
ranged; fuch as pincers, files, turnfcrews, compaffes, gim- 
blets, wire, needles, thread, and packthread. 
15. Laftly, fome good map, pafted on canvas, of the coun- 
try you propofe to examine; and this map ought frequently 
to be compared with your itinerary, and the bearings given 
by your compafs, 
16, In regard to the care required for the traveller’s perfon, 
he muft have a light drefs made of cloth, without lining, of 
a white colour, as well as his hat, that he may be lefs ex- 
pofed to the heat of the fun’s rays; with jackets, fome cool for 
the warm regions and the vallies, and the other warm for 
the cool regions and eminences; a good great coat; green 
fpeGtacles, and a black crape, to fecure the eyes and face from 
the fuow. Laftly, if he is to pafs the night in the open air, a 
* Scétion 13097, Plate 1, fig. 3. 
Ef. tent 
