trying to set a parameter equal to an elements attribu

when I apply the following transformation I get a blank output. can anyone
tell me what I am doing wrong?

<!-- XML FILE -->
<root>
    <header volume="8"/>

    <article>
        <para>
            Some text here
       </para>
    </article>
</root>

<!-- XSLT FILE -->
<xsl:stylesheet version="1.0" xmlns:xsl="">
    <xsl:template match="root">
        <xsl:param name="vol"><xsl:value-of select="header[@volume]"/>
    </xsl:template>
</xsl:stylesheet>




Thanks
Noel


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