Measurements have been made at Durham (200ft. above sea level) with an emulsion spectrograph of the absolute cosmic ray particle intensity in the momentum range 1 to 100 GeV/
c
at zenith angles of between 65° and 85°. It is found that a differential π-meson production spectrum of the form
I
0
E
-y
fits closely the present results as well as the accepted vertical spectrum. The values of the parameters when
E
lies between 6 and 1000 GeV are given by the equations
I
0
= 0.425 — 01.25 log
10
E
and
y
= 3.92 — 0.944 (1 — 0.125 log
10
E
)
-1
and their approximate constant values in this range are 0T5 and 2*55 respectively. The analysis is based on the model of Barrett, Bollinger, Cocconi, Eisenberg & Greisen (1952) but, in addition, the effects of scattering and geomagnetic deflexion of u-mesons in the atmosphere have been taken into account as well as their production over a range of atmospheric depths. Although in principle it should be possible from the form of the sea-level spectra at large zenith angles to determine the relative numbers of π- and K -mesons at production, it is shown that very great accuracy is required in order to do so. Nevertheless, the agreement between the theoretical curves and the experimental measurements indicates that the main assumptions concerning the production and propagation of mesons in the atmosphere are correct. The mean positive to negative ratio is 1.39±0.08, a value rather larger than is found by other observers at 68°, but within statistical fluctuation.
