Dr. Arnoxt on the 3Ieasuremenis of Heights. 9 



difference of the logarithms of the pressure, — that is, they were propor- 

 tional to the differences of the approximate heights obtained by means of 

 a barometer ; that a constant multiplier must, therefore, give the approxi- 

 mate height ; but that it would appear that Dr. Thomson had not applied 

 the very important corrections for the temperature of the air to that 

 approximate height, and which are as necessary for the thermometer, as 

 when the barometer is used. At the same time I pointed out, that 

 although the discovery of the above la,w, and the application of a constant 

 multiplier, was now usually ascribed to Professor Forbes of Edinburgh, 

 both were, at least twenty-five years previously, adopted by the late Sir 

 John Leslie, in the [supplement to the 5th edition of the Encyclopaedia 

 Britannica, article Barometrical Measurements, which article was after- 

 wards inserted in the 7th edition of that work ; the former was published 

 in the year 1817 or 1818, the latter about 1830. Sir John Leslie also 

 taught the same method in the Natural Philosophy class, — at least he did 

 so during the winter 1816-17, when he undertook the management of 

 that class during the absence of Professor Playfair in Italy, and illustrated 

 it by many cuiious examples which have not been printed, but which are 

 referred to in my notes, taken whUe attending his lectures that winter. 

 Professor Forbes says, (Trans, of Roy. Soc. of Edinburgh, xv. p. 413,) 

 " It is singular that so elegant and so simple a result should have escaped 

 every writer on the subject, (so far as I know,) even Deluc himself, who 

 proposed the logarithmic law, and Wollaston who, unawares, adopted the 

 true law as a first approximation, and then took a wrong one :" from 

 which it would appear that Jie had never read his predecessor's article on 

 the subject, or that at the time it had escaped his recollection. Professor 

 Forbes arrives at his conclusion by means of some very valuable observa- 

 tions made by himself in Switzerland, — Leslie considered it as already 

 established by Saussure's observations and Deluc 's logarithmic law. 



As Sir John Leslie's views seem to be little known, or forgotten, I shall 

 here insert them : — 



" The heat at which water boils or passes into the form of steam, 

 depends on the weight of the superincumbent atmosphere. By diminish- 

 ing this pressure, the point of ebullition is always lowered. It appears 

 that while the boiling point sinks by equal differences, the corresponding 

 atmospheric pressure decreases exactly, or at least extremely nearly in a 

 geometiical progression: it being found that every time such pressure is 

 reduced to one-half, the temperature of boiling water suffers a regular 

 diminution of about eighteen centesimal degrees. This beautiful relation 

 assimilates with the law which connects the density and elevation of the 

 successive strata of the atmosphere. TJie interval noticed between tJie 

 hoilin'j paints at tv;o distinct stations must he proportional to their difference 

 of altitude above the level of the sea. We have therefore only to deter- 

 mine the co-efficient or constant multiplier, which may bo discovered, 

 cither from an experiment under the rarificd receiver of an air pump, or 

 from an actual obseiTation performed at the bottom and at the top of 



