Use jquery property "slideDown"
function hideshow() {
var i = 1;
if (document.getElementById('effet').style.display == "none") {
$('#effet').slideDown("normal");
document.getElementById('effet').style.display = "block";
i = 2;
} else {
if (i == 1) {
$('#effet').slideUp("normal", function () {
document.getElementById('effet').style.display = "none";
});
}
}
}