THE MAKERSTOUN OBSERVATIONS. » 



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 the instrument in observatories, the only one from which laws 

 of variation might be expected with little trouble, yet the observations have 

 in general been overrun 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 gumming 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 Kere my doubts 

 as to the accuracy of the theory of the wet bulb, even as a measurer of the liumidity 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 obtained in the mean for the 

 year ; which, when subducted 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 Report into the Results 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 written, that Colonel Sykes has also, in a paper read 

 before the Royal Society of London last session, objected to the results from the psychrometer. 



