Intelligence and Miscellaneous Articles. 135 



This being the case, let us, at the time of full moon, choose a 

 place of observation sufficiently open, but where there is at least one 

 wall, illuminated either by the moon or by street-lamps. If the 

 sky is clear, let us keep our eyes fixed for some time upon one of 

 the spots of the luminary, situated near its centre, then turn quickly 

 towards the wall in order to project upon it the dark accidental 

 image of the lunar disk. If that image appears to us smaller than 

 the moon itself, let us more further from the wall, but nearer to it 

 if. on the contrary, the image appears larger, and recommence the 

 experiment, until we judge the two diameters equal. This equality 

 evidently demands that we refer the accidental image to the same 

 distance as the luminary ; therefore in order to get the distance to 

 which we refer the moon, we shall then have only to measure that 

 which separates us from the wall. 



Only I must here note some more or less influential causes of error. 

 In the first place, the exact appreciation of the equality of the dia- 

 meters of the image and the luminary is, one can understand, very 

 difficult: for the two objects cannot be observed simultaneously. 

 In the second place, we may be mistaken in the idea we form of our 

 distance from the wall, especially if there are no trees or houses to 

 serve as intermediaries. In the third place, clouds floating in the 

 vicinity of the moon would doubtless modify the involuntary judg- 

 ment we give on the magnitude, and consequently on the distance. 

 of the luminary. It is moreover probable that the distance, esti- 

 mated in the way we have indicated, would vary with different 

 persons. 



My second son, whose sagacity in observation I have proved on 

 many occasions, carried out the experiment under the following 

 conditions : — The house in which I reside looks towards the south ; 

 it forms part of one of the long sides of a rectangular " square,"' 

 part of one of the short sides of which is formed by the wall of an 

 enclosure. On the 23rd of April, the eve of the full moon, at ten 

 o'clock at night (*. e. one hour before the moon's passing the meri- 

 dian) the sky was perfectly clear : and when my son placed himself 

 against our house, he saw the luminary shining in all its splendour 

 at a sufficient altitude above the houses on the opposite side of the 

 square. But as the presence of these might have some influence, he 

 held his hand so as to hide them and them only. After looking at 

 the moon for a sufficient time, he looked at the enclosure--^ all of 

 which I have spoken, which was lighted by the lamps of the square ; 

 and he moved nearer to or further from it, viewing the moon afresh 

 when the dark image vanished, in order to get the proper distance. 

 For the purpose of determining it with the least error possible, he 

 walked towards the wall till the dark image appeared to him deci- 

 dedly a little smaller than the moon, then retreated until it appeared 

 decidedly a little larger : and he took the middle point between 

 these two extremes for that which most probably fulfilled the desired 

 condition; moreover the houses on the side of the square near 

 which he was opera tins: constituted intermediaries suitable for 

 giving him a sufficiently distinct idea of the distance of the wall on 



