142 Professor Leslie's Remarks on the Instruments 



and, 11. The drosometer, which measures the quantity of dew. 

 These various*instruments are not, however, all of equal impor- 

 tance. The barometer, the thermometer, and the hygrometer, 

 may be considered as quite indispensable. Next to them, de- 

 serves to be ranked the photometer and aethrioscope, which dis- 

 close the more recondite condition of the atmosphere. The 

 atmometer, the ombrometer, and the anemometer, are of great 

 consequence, from the practical results which they furnish. I 

 would strongly recommend, as a most useful auxiliary in meteo- 

 rological observations, Rutherford's maximum and minimum 

 thermometer. In many cases, likewise, it would be convenient 

 for^ the scientific traveller to be provided with a thermometer 

 bearing large divisions, and lodged at the bottom of a walking- 

 stick, protected by a coating of down inclosed within a brass 

 tube. This instrument is peculiai'ly adapted for exploring the 

 temperature of the ground and of springs*. 



But the value of any meteorological register must depend on 

 the accuracy with which it is kept. The observations should 

 be made in a place rather elevated, sheltered from the direct ac- 

 tion of the sun, but exposed freely on all sides to the aspect of 

 the sky ; and they should be repeated either at equal intervals, 

 during day and night, or at least at those hours which represent 

 most nearly the mean state of the atmosphere. These requisites 

 are seldom attained, and very few registers of the weather, accord- 

 ingly, are entitled to much confidence. 



It cannot be expected, that registers of the weather will pos- 

 sess much value, so long as they are kept merely as objects of 

 curiosity. Like astronomical observations, as now conducted, 

 they should no longer be left to the chance of individual pur- 



• It would be particularly desirable, if travellers over land were provided 

 with light barometers and stafF-thermometers. A very portable barometer, 

 sufficiently accurate for general purposes, might be constructed with a conical 

 tube, or two portions of unequal diameters conjoined. But the stafF-thermo- 

 meter might often supply the want of a barometer, by discovering the mean 

 temperature at moderate depths under the surface. Hence the relative alti- 

 tudes of different places above the level of the sea could be estimated with 

 tolerable precision. Had the various travellers who have visited the Interior 

 of Africa made observations of that kind, the question respecting the course of 

 the Niger would have been decided long before now ; at least we should have 

 known, whether the great lakes were, like the Caspian, below the surface of 

 the ocean. 



