TRANSACTIONS OF THE SECTIONS. 47 



by tlie Astronomer Royal in the ' Proceedings of the Meteorological Society,' vol. 

 iii. page 406, and gives results which lie between those of other authorities. 



Aneroids constructed for tlie determination of elevations by readings from analti- 

 tude-scale consist of two classes — one in which the altitude-scale is fixed and the 

 other in which it is movable. The first class of aneroid, with a fixed scale, is 

 accurate in principle ; but the scale only allows for one of the conditions which 

 have to be taken into account, viz. the varying pressure of the atmosphere ; and the 

 other condition or temperature of the atmosphere has to be allowed for by calcula- 

 tion. The second class of aneroid, that with a movable scale, is radically wrong 

 in principle as ordinarily used, inasmuch as the movable scale must be graduated 

 for one fixed position of the zero ; and when the zero is shifted at random, accordino- 

 to the position of the hand of the instrument, the scale necessarily becomes inac- 

 curate. 



In the improved aneroid the scale of altitudes is movable, but, instead of being 

 shifted at random according to the position of the hand of the instrument, it is 

 moved into certain fixed positions according to the temperature of the atmosphere ; 

 so that the shifting of the scale answers the same purpose as if the original scale 

 were altered to suit the various temperatures of the atmosphere. The aneroid is 

 graduated for inches in the usual way on the face ; but the graduation only extends 

 from 31 to 27 inches, so as to preserve an open scale. The outer movable scale is 

 graduated in feet for altitudes; and the graduation is laid down by fixing the zero 

 opposite .31 inches. This is the normal position of the scale ; and it is then correct 

 for a temperature of 50° Fahr. For temperatures below 50° the zero of the scale is 

 moved below 31 inches ; and for temperatures above 50° the zero of the scale is 

 moved above 31 inches : the exact position of the zero for diiferent temperatures 

 has been determined partly by calculation, and partly by trial, and marked on the 

 rim of the aneroid. In order to ensure the altitude-scale not being shifted after it 

 has once been set in its proper position, there is a special contrivance for locking it 

 in the various positions. The altitudes are in all cases determined by taking two 

 readings, one at each station, and then subtracting the reading at the lower station 

 from that at the upper. 



The movable scale requires to be set for temperatures before taking any obser- 

 vation, and not shifted during the progress of the observations. This wiU practi- 

 cally not give any inconvenience in the case of moderate altitudes, as small 

 variations of temperature will not appreciably affect the result ; and so long as the 

 temperature does not vary during the course of the observations more than 5° 

 from that at which the instrument is set, the result may be accepted as practically 

 correct. 



In conclusion the author states that the principle of allowing for the variations 

 of temperature of the atmosphere by shifting the altitude-scale, does not profess to 

 be theoretically accurate, but simply sufficiently accurate for practical purposes. In 

 order to satisfy himself that this was the case, the author carefully compared 

 the readings obtained for diilerent temperatures from the shifted scale with the 

 correct readings as given by calculation from the normal position of the scale 

 and found that the maximum error was 2 feet and the average error under 1 foot' 

 errors which are perfectly inappreciable. The instrument was constructed by 

 Mr. Casella, of Ilolborn Bars, London. 



On Eckhold's Omnimeter, a new Surveying-Instrument. By G. "W. Hope. 



On Negretti and Zambia's Test-gauge Solar-Radiation Thermometer. 



By G. J. Stmons. 



Meteorologists have long been endeavouring to obtain an instrument whereby 

 comparable observations of the amount of solar radiation could be made. Various 

 experiments and observations by the Rev. F. W. Stow, the late F. Nunes, Esq. 

 M.A., and the author have shown that this object is attained by the use of a mer- 

 curial maximum thermometer, of which the bulb and one inch of the stem are 



