THE MAKEESTOUN OBSERVATIONS. 
9 
ably lower temperature than in the shade elsewhere.* The thermometer case 
should, I think, be placed to the north of the building, not in a recess, but sheltered 
from the sun's rays in the morning and evening by large double boards placd at a 
moderate distance, and permitting a free circulation of the air : the case itself 
should be similar to that employed at Makerstoun, or of a lighter construc- 
tion, which can be turned round from within the observatory, so as to allow the ob- 
servations to be made from one of the windows, the glass of which prevents the ef- 
fects of radiation from the body of the observer, or from the lamp during night. 
The Declinometer. — This instrument, which is of the simplest possible con- 
struction, requires no correction for the effects of temperature ; it has generally, 
therefore, been tlie instrument in observatories, the only one from which laws 
of variation might be expected with little trouble, yet the observations have 
in general been oveiTun with errors. The errors of the instrument are due 
chiefly or altogether to the suspension of the magnet. 
The French Academy of Sciences thought the mode of suspension of the de- 
clination-magnet of so much importance, that they offered it as the subject of a 
prize in the year 1777 ; previously the needle was balanced upon a pivot as in 
our common mariner's compass. The prize was carried by Coulomb, who pro- 
posed suspension by means of a thread formed of the silk fibres from the 
cocoon. This suspension was adopted immediately after by M. Dominic Cassini ; 
the old cap and pivot suspension was used, however, for some time afterwards, 
as by Gilpin, early in the present century. Cassini seems to have soon become 
aware of the effects of humidity, and of separation of the fibres, in introducing 
torsion into the thread ; for he prepared his threads by first gumnnng the fibres 
together, and then greasing the thread. It is obvious, however, that, after all, 
* As I will not again allude to the meteorological instruments, I may mention here my douhts 
as to the accuracy of the theory of the wet bulb, even as a measurer of the humidity of the locality 
in which it is placed ; and my perfect belief that the determinations of the vapour pressure in the 
atmosphere, obtained by means of the dry and wet bulbs, are wholly in error. I have indicated this 
opinion in different volumes of the Makerstoun Observations, where I have also shewn that the ap- 
parent success of an attempt made by M. Dove, and lately by Colonel Sabine, to resolve the diurnal 
variation of the total pressure of the atmosphere into two simple variations, is in all probability due 
to the large but erroneous diurnal variation of aqueous vapour pressure obtaiired in the mean for the 
year ; which, when stibducted from the smaller double variation of the total pressure, leaves traces 
only of its own abstraction. I have shewn that this is the more certainly true, since, when we con- 
sider winter only, the variation of the aqueous vapour pressure, as computed, being then very small, the 
pressure of the dry air then exhibits a well marked double diurnal variation as before. I do not 
enter in this Heport into the Eesults of the Makerstoun Observations. I do not mention the pre- 
vious fact to shew that the pressure of aqueous vapour is not involved in the diurnal variation ob- 
tained from the barometer, but as some evidence that the pressures deduced from the dry and wet 
bulbs are not to be trusted. I have proposed and attempted some experiments for the purpose of 
obtaining the actual value of the vapour pressure in the atmosphere, by destroying the moisture in 
a closed apparatus, but have not yet succeeded, owing to imperfections in the instrument. 
I am glad to learn, since the above was wa-itten, that Colonel Sykes has also, in a paper read 
before the Eoyal Society of London last session, objected to the results from the psychrometer. 
C 
