VOL. LXXXI.] PHILOSOPHICAL TRANSACTIONS. 3? 



The re-union of the 2 measurements, of latitude and longitude, in the same 

 spot, is an advantageous circumstance; and the more so, if we consider that 

 this spot lies between the 45 and 46th degree, that is, in the mean latitude be- 

 tween the pole and the equator, near which latitude the mean radius of the 

 earth takes place in the well-founded supposition of its being a spheroid. This 

 radius, found by the most accurate measurement hitherto attempted, would be- 

 come a standard, and to which the results of the equatorial and northern mea- 

 surements being compared, the true figure of the earth would be the better 

 ascertained. If a survey of this kind, executed in a mountainous country, is 

 liable to some difficulties, it offers, on the other hand, advantages which perhaps 

 more than overbalance those difficulties. First, the visual rays being less inter- 

 rupted, the triangles become larger, and the stations fewer in number; whence 

 the labour of the observers, and the chances of error, are by so much dimi- 

 nished. 2dly. These same visual rays proceeding through strata of air less dense 

 and more free from the vapours which commonly thicken the lower parts of the 

 atmosphere, the danger of irregular refractions is by so much less, and the sig- 

 nals may be more distinctly perceived from great distances. 



Besides those advantages which chiefly concern the measurement itself, the 

 country would offer facilities for other natural inquiries, not unworthy the at- 

 tention of philosophical men, and which might easily be united with the capital 

 object of these labours, with which object some of these inquiries are intimately 

 united. Among them Mr. P. places, the accurate determination of the length 

 of the simple pendulum, which beats seconds in this mean latitude. Experi- 

 ments to be made on the oscillations at different heights, with an invariable pen- 

 dulum. Experiments on the lateral attraction of mountains repeated and varied. 

 Observations on meteors, and several atmospherical phenomena relative to re- 

 fractions, to heat, to hygrometry, to electricity, &c. 



But, above all, the improvement of barometrical measurements would mostly 

 deserve the attention of the commissioners. Nothing, as it seems, is now want- 

 ing in the theory of the operation; and it is only from a number of actual ob- 

 servations, made at different heights, and with every due precaution, compared 

 with geometrical measurement, that the co-efficient, either constant or variable, 

 to be applied as a correction for the atmospherical heat, will be obtained. That 

 research must be merely empirical; the effect sought for being the result of many 

 complicated causes, some of which are yet unknown. The real height of every 

 station being well determined, the time to be spent there for other purposes 

 would allow a number of barometrical observations in varied circumstances; and 

 from these observations, rightly compared between themselves, interesting and 

 useful results may justly be expected. 



