520 
THEORY OF THE WET-BULB HYGROMETER. 
tions applied in order to arrive at 
accurate results. I shall therefore only 
remark, that the method by which the 
intensity of the magnetic force was 
ascertained, resembles in principle that 
by which philosophers determine the 
force of gravity. For as a pendulum 
when set in motion oscillates on either 
side of the verticle line by the force 
of gravity, so the needle, when 
drawn out of its natural position, 
will oscillate on either side of the 
magnetic meridian by the earth’s 
magnetic force, and hence, in either 
case the force may be inferred to 
vary, inversely as the square of the 
time in which a certain number of vi- 
brations are performed. In order, 
however, to arrive at trustworthy re- 
sults, many precautions must be adopt- 
ed, which are pointed out in detail in 
Professor Lloyd’s memoir, and in par- 
ticular one relating to temperature ; 
it being found that the same needle 
will vary in force about 1-4 000th part 
for every degree of Fahrenheit. Hav- 
ing, however, arrived at a determina- 
tion of the intensity of the magnetic 
force at the two extremities of the Is- 
land by a sufficiently extended series 
of observations, namely, at Limerick 
by Captain Sabine, and at Dublin by 
Professor Lloyd, and having compared 
the results with those obtained by 
means of the same needles at a spot out 
of Ireland, whose magnetic intensity 
had been previously settled, by avail- 
ing themselves of the observations of 
Captain James Ross, at London, our 
authors proceed to estimate the rela- 
tive intensity of the magnetic force at 
twenty- five different places within the 
compass of Ireland, by observations 
made at each of these simultaneously 
with others at Dublin or at Limerick. 
They thus obtained data by which to ex- 
hibit the law of I'errestrial Magnitism in 
Ireland, in a similar manner to that by 
which Humboldt laid down the laws 
of the distribution of Terrestrial Heat. 
The same principal was adopted in 
determining the lines of dip as of in- 
tensity, and the general result was ob- 
tained, that the angle which the lines 
of dip in Ireland make with the meri- 
dian of Dublin is 560 48’, and that the 
dip increases one degree for every dis- 
tance 10 of miles in a direction perpen- 
dicular to these lines. 
The preceding method of estimating 
the intensity by the number of vibra- 
tions in a given time, only applies to 
that portion of the earth’s magnetic 
force which operates in a horizontal di- 
rection. In order, , therefore, to deter- 
mine the whole amount of this force, 
ovservations, of the kind above alluded 
to, must be combined with others on the 
dip. This third series accordingly was 
instituted at twenty-three different sta- 
tions in Ireland, and the result arrived 
at was, that the lines of absolute inten- 
sity make an angle of 330 40]| with the 
meridian of Dublin, and that the inten- 
sity increased in a direction perpendi- 
cular to these lines by the 1-lOOth part 
for every 95 miles of distance. 
The importance of these researches in 
extending our knowledge of Terrestrial 
Magnetism, and affording the data on 
which a correct theory with respect to 
this subject may hereafter be based, will 
be felt even by those who do not fully 
appreciate the skill and labour they re- 
quired, and no better proof could be 
afforded of the substantial benefits aris- 
ing from such an institution as the Bri- 
tish Association, than that of having 
originated such an inquiry 
On the subject of Heat, Dr. Hudson, 
of Dublin, has detailed some experiments 
the tenor of which he considers incom- 
patible with the commonly received the- 
ory respecting its radiation, which we 
owe to Professor Prevost, of Geneva, in- 
asmuch as their tendency would be to 
establish that cold is equally radiated 
with heat — a result inconsistent with the 
notion of the former being a negative 
quality. He consequently leans rather 
to the views of Professor Leslie, who 
supposed heat to be radiated in conse- 
quence of the alternate expansion and 
contraction of the air around, producing 
a series of aerial pulses. 
In compliance with a wish expressed 
by the MeteoroLgical Committee, Dr. 
Apjojin has investigated the theory of 
the Wet-bulb Hygrometer, and commu- 
nicated an account of his experiments 
on this subject at the Dublin Meeting. 
His paper, having been already publish- 
ed in the Iransaction of the Dublin 
Academy, does not appear in our Re- 
port, which, however, contains two very 
interesting communications on subjects 
of Meteorology. 
Mr. Snow Harris has presented a 
statement of the variations of the ther- 
mometer at the Plymouth Dock-yard, 
