624 Lieutenant Rennt on the Constants of 



constant by a considerable number of observations, hygrometric as well as baro- 

 metric, it was my opinion (an erroneous one) that in this way only such con- 

 stant was to be determined. I made arrangements, therefore, early in the year 

 1857, for such observations, during my residence at the head of Lake Geneva, 

 near to the Castle of Chillon. In a work of this kind the first thing to be done 

 was evidently to obtain, by means of accurate spirit-levelling, the heights of 

 various stations above a common starting-point, the height of such starting- 

 point above the level of the sea being known, if not with the precision obtained 

 by spirit-levelling, at all events approximately. Both these objects have been 

 attained satisfactorily : the first, by my own personal exertions, working with 

 the very best instruments of London manufacture ; the latter, by the labours 

 of civil engineers, French and Swiss, who have connected the level of 

 Lake Geneva with the level of the sea at the Gulf of Lyons, by accurate spirit- 

 levelling. 



My own starting-point was the central one of three iron bars, connected 

 with the Limnimetre of Geneva, at the north-west foundation stone of Chillon 

 Castle. My highest point is upwards of 1000 English feet above the founda- 

 tion stone of Chillon Castle, near to a new hotel, called " Pension Eigi Vau- 

 dois," immediately above the village Montreux. 



Several intermediate points have been determined by me with peculiar care, 

 and connected with the extreme points of the levelling operations. 



It will, I believe, be satisfactory to the Academy to be made acquainted 

 with the peculiar helps and faciUties which I enjoyed in my labours near 

 Montreux. 



I possessed two excellent mountain barometers, made by ]\Ir. Newman, 

 Regent-street, London ; a wet and dry bulb hygrometer, made by Ronketti, 

 Great Russell-street, London ; also a Gravatt level of London manufacture ; 

 and I was assisted by M. A. Morlott, ci-devant Professor of Geology at the 

 College of Lausanne, a young man of high scientific attainments. He also 

 possessed an excellent syphon mountain barometer, together with a wet and 

 dry bulb hygrometer, both of Vienna manufacture. 



At the commencement, and for some time afterwards, of the joint labours of 

 M. A. Morlott and of myself, I frankly confess I did not entertain much 

 hope of being able to make a number of observations sufiiciently great to be 



