96 EFFECTS OF FORESTS ON MOISTURE. 



tions all eirors made are errors of deficiency, arising from the escape 

 of water by evaporation, by infiltration, by spilling, and from neglect, 

 etc. ; and doubtless the first observers did not attach any great im- 

 portance to those measures "which, in the absence of other correspond- 

 ing observations made elsewhere, could not lead to any generalization. 



"Thus Father Cotte, quoted by M. Raulin, writes in 1804: — 'I 

 may observe that the udomfetre used at the Observatory of Paris has 

 been found to be so defective that they have been obliged to give up 

 the use of it for some years ; they have now set up a new one, which 

 is much more exact. The water is received and measured on the 

 platform of the building, 27 metres [90 feet] above the ground, instead 

 of measuring it, as formerly, in one of the apartments on the first 

 storey, so that it ran through a pipe 63 feet long from the platform 

 where it was received to the reservoir, which necessarily occasioned 

 a loss.' 



" From these diflferent causes it is impossible to compare satis- 

 factorily the series of observations from 1688 to 1754 with the series 

 made from 1803 to 1860. It is possible that if the two series of 

 observations had been made with the same instrument, and with the 

 same precision, the latter might have shown a diminished instead 

 of an augmented mean. 



"Arago said, on this subject, in 1853*: — ' There is no reason to 

 suppose that the climate of Paris is either more or less rainy than it 

 was 150 years ago. The small augmentation presented by the 

 numbers in the later groups do not in fact exceed variations observ- 

 able in the earlier periods.' 



'* It is known that in 1817 a second udometre was placed on the 

 ground of the court, and that ever since it has constantly received 

 quantities of water a Httle in excess of what was received by that on 

 the terrace, as may be seen from the following numbers : — 



Measurement of Water Annually 



Period. Received Proportion. 



On the Terrace. On the Ground. 



a b a-b. 



miUimt'tres. mm. mm. 



1821-1830 498-6 551-9 MO 



1831-1840 509-3 584-8 1-14 



1841-1850 529-3 627-8 MS 



1851-1860 620-3 577-1 1-40 



* Arago : " CEuvres completes Melanges (p. 420j. 



