SI 
own that might or might not coincide with the atmospheric one, 
likewise the change in the eccentricity. 
It remains to consider whether the time necessary for these 
changes falls within working limits. If we take Reade’s estimate 
of the amount of CO2 in the atmosphere and the amount annu- 
ally consumed in bicarbonates we find that it takes only 926 years 
to exhaust half of the CO2 in the atmosphere now and only 
17,500 years to exhaust half of the CO2 of the atmosphere and 
the sea. There is a considerable variation in the estimates of the 
length of time involved in and since the recent glacial period, but 
all the estimates that have met with any favor are consistent with 
the figures given, viz., 17,500 years. 
ASPAMILY OF BLU EABIRDS: 
PRINCIPAL JOHN D. WILSON. 
JUNE 14, IQOT. 
The scarcity of suitable nesting places is a cause for the dim- 
inution of our native birds, which has not received the attention 
which it deserves. 
Sparrows are versatile architects. They can nest anywhere. 
That is why they breed so fast. 
Hollow stumps and trees which formerly furnished homes 
for blue birds, wrens, and chimney swifts, are rapidly being 
cleared away. ‘The place of these might be easily supplied by 
artificial devices which would largely increase the number of 
birds in a very few years. ‘Those who have never tried the exper- 
iment will be surprised at the readiness with which birds respond 
to assistance. 
‘Early this spring, the recollection that, when a boy, I had 
often found a blue bird’s nest in a hollow stump led me to make 
an attempt to entice a pair to locate in my garden. 
On the t2th day of April I took four pieces of common 
house-siding about four inches wide and nailed them in the shape 
of a hollow, square prism. ‘The opposite sides were respectively 
twelve and fifteen inches in length. I nailed a little strip across 
