ON ITS EXTERIOR. VOLCANOES. 347 



Monticelli and Covelli found for the Rocca del Palo 

 3990 feet, and I found nearly 4022 feet. I assumed 

 as the most satisfactory final result for that time 3997 

 feet. In the spring of 1855, therefore, thirty-three years 

 later, the fine barometrical measurements of the Olmutz 

 astronomer, Julius Schmidt, again gave 3990 feet. (Neue 

 Bestimm. am Vesuv., 1856, S. 1, 16, and 33.) How 

 much of these differences is due to error of observa- 

 tion, barometric formulae, and casual circumstances inci- 

 dental to the method? Such investigations could be 

 carried on more extensively and securely, if, instead of 

 often repeated complete trigonometrical operations, or, 

 in the case of accessible summits, of the more easily 

 applied, but less satisfactory, barometric method, it 

 were arranged simply to take, at desirable intervals, 

 say twenty-five or fifty years, angles of altitude of the 

 crater margin from some definite and easily identified 

 spot, but making those angles exact to fractions of 

 seconds. On account of the influence of terrestrial 

 refraction, I would advise that, at each normal epoch, 

 the mean of observations at several different hours and 

 on three different days should be obtained. In order 

 to have not merely the general result of increase or 

 diminution, but also the absolute amount of change 

 expressed in feet, it would suffice to determine, once for 

 all, the distance of the point of observation. What a 

 rich source of knowledge, founded on experience extend- 

 ing over more than a century, respecting the colossal 

 volcanoes of Quito, would now be open to us, if, in addi- 

 tion to the sufficiently exact measurements left to us by 

 Bouguer and La Condamine, those distinguished men 

 had furnished us with the knowledge of exact spots, per- 



