ON THE MAGNETIC INTENSITY OF THE EARTH. ré 
served appears to have been 77, all of which are entered in 
the general table in this memoir. 
Besides the land-stations, there are 12 geographicai posi- 
tions, in which M. de Humboldt observed the vibrations of the 
needle on board ship. There are two great and obvious dis- 
advantages in such observations, compared with those on land, 
viz. the motion, and the iron, of the vessel. On the other side 
should be noticed, the space interposed between the instrument 
and the solid materials of the earth’s surface, many of which 
are known to exercise a very considerable disturbing influence 
on the needle. As opinions may, and I believe do, vary in re- 
gard to the degree of relative value to be allowed to observa- 
tions of intensity made at sea and on land, and as it is not a 
point on which, from personal experience, I fee] qualified to 
decide, I have placed the sea-observations in a separate table, 
and subjoin them here. 
Latitude. Longitude. Date. Intensity. 
° 
3s 62 | 34559 | 1799 | 1-315 
37 26 | 345 49 | 1799 | 1-315 
34 30 | 34526 | 1799 | 1-230 
31 46 | 34517 | 1799 | 1-261 
24 53 | 34123 | 1799 | 1-283 
3028.) 279 54 | 1803 | 1-067 
2129 | 33639 | 1799 126181 1956 
19 54 | 33336 | 1799 | 1-251* 
1415 | 31418 | 1799 1-285 | 1.050 
13 02 | 30923 | 1799 | 1-230* 
10 46 | 30127 | 1799 | 1-178" 
11 01 | 297 30 | 1799 L261 } 1220 
The results marked with an asterisk were observed on the 
passage across the Atlantic, between Teneriffe and Trinidad, a 
part of the ocean where no land exists, and where, consequently, 
the results obtained at sea furnish the only attainable evidence. 
On examination, they present differences among themselves 
considerably greater than is usual in land results ; but by com- 
bining them in pairs, as shown in the table, and using the mean 
latitude, longitude, and intensity of each pair, these partial dif- 
ferences greatly disappear. I have entered the mean latitude, 
eppinde. and intensity of these three pairs in the general 
table. 
Humboldt and Gay Lussac, 1805-1806.—These observations 
