THE AUGUST METEORS. 



183 



for one observer. His results are compared with a similar average de- 

 rived by Major Tup man and the writer from observations in 1869-71 

 and 1877-80 respectively : 



Schmidt's figures are very small and much below the numbers 

 found in recent years. But the averages in the table are not thorough- 

 ly reliable, inasmuch as they are based upon only a few years' observa- 

 tions. A longer series might give a closer comparison, but it is seldom 

 that the results of independent observers agree within small limits. 

 There are differences in vision, modes of observation, and in position, 

 which must obviously affect the numbers to no small degree ; and the in- 

 termittent character of the meteor-shower itself must give rise to discrep- 

 ancies which can not at first sight be accounted for. The horary num- 

 ber of meteors on August 10th may vary, according to Heis, from 160 (in 

 1839) to 24 (in 1867). During the last ten years the writer has found 

 little variation in the intensity of the annual returns when the condi- 

 tions of weather and moonlight are fully taken into account ; and 

 there is no question that some of the variations ascribed to the shower 

 have no real existence, but are to be explained by the differences re- 

 ferred to above. 



A fair comparison can not be instituted between the horary num- 

 bers found by observers, unless the observations, from which the values 

 are deduced, are made, in each case, at similar hours of the night ; for 

 shooting-stars, though often plentiful after midnight, are comparative- 

 ly scarce in the evening hours. This is readily explained by the fact 

 that the principal radiant points of the showers are massed together in 

 the eastern region of the sky where the earth's orbital motion is direct- 

 ed, and it is obvious that in the evening hours, when the altitude of 

 many of them is very low, and when others have scarcely appeared above 

 the horizon, their operation is in a great measure restricted, so that 

 only a feeble indication of their displays is perceptible at such a time. 

 The case is entirely different at a later period of the night, when the con- 

 stellations in which the several radiant points are situated have ascend- 

 ed high in the sky, and are in fact so placed that they may be seen to 

 the greatest advantage. The August Perseids are always best observ- 

 able in the morning hours, for the radiant point is very low on 'the 



