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172 EXPLORATIONS ACROSS THE GREAT BASIN OF UTAH. 



had shortened the scale 0.028 inch, as indicated by a mark on the brass tube. The 

 apparent error, after the insertion of the new tube, ought, therefore, to have been 0.064 

 minus 0.028, equal to 6.036 inch. That the direct comparison gave it a little larger, 

 0.042, may be accounted for by my inability to measure the inner diameter of the 

 tube to a fraction, as the beautifully clear sound of the instrument (produced when 

 the mercury struck the closed end of the tube) indicated that the vacuum was perfect. 

 The result* certainly was very satisfactory. It proved that the zero-errors of the 

 instruments had bJen recorded correctly,^ or very nearly so. I might, then, have 

 shortened the scale, as the maker would have done in a similar case, being satisfied 

 lequenee of a fault of the instrument, but of the 



ened to the two instruments, which, at the time, 

 l Some of the mules got entangled in the cords 

 bw the tripod, with both barometers, to the ground. 

 2, and rendered it temporarily unserviceable. A 

 f No. 1279, but left it again on turning the instru- 

 ct not suffered anv permanent damage. The sound 

 lie vacuum was still perfect. After the tube of No. 

 ible precaution, I found the result of calculation 

 t of the direct comparison of the instruments, and 

 in this way I was again re-assured that No. 1279 had not suffered from the accident, 

 an assurance which I could not well have arrived at in any other way. These 

 examples show how useful it is to keep account of the width of the tubes, the height 

 of the meniscus, the clearness of the sound, and other observations in regard to the 

 condition of the instruments. 



The thermometers — the attached as well as the detached ones — also did not per- 

 fectly agree with each other. I therefore tested their graduation by direct experiments, 

 from which I calculated a table of corrections. The readings of the thermometer, as 

 found in the records, were thus corrected whenever it was found necessary. 



I could scarcely hesitate in the selection of the method for computing the altitudes, 

 since the one developed according to the requirements of the ease during the compu- 

 tation of the profiles of the Pacific Railroad surveys, and discussed by Lieut. Henry 

 L. Abbot, Topographical Engineers, in Vol. VI of the Reports (to which I refer for 

 particulars), gives results which may be regarded as absolutely correct, as demon- 

 strated by Lieutenant Abbot, if suitable corrections can be obtained; and under less 

 favorable circumstances, the results are at least more generally reliable than those 

 obtained in any other way. By the introduction of the corrections for horary and 

 abnormal oscillations of the barometric column, if such can be obtained from points 

 of similar climatical features, not too far distant, nor differing too much in altitude 

 from the point the altitude of which is to be determined, all causes of error are elim- 

 inated the more the nearer these conditions are fulfill -\. including the effects of the 



